Glucose sensor electrode design

ABSTRACT

A single flex double-sided electrode useful in a continuous glucose monitoring sensor. In one example, a counter electrode is placed on the back-side of the flex and a work electrode is placed on the top-side of the sensor flex. The electrode is fabricated on physical vapor deposited metal deposited on a base substrate. Adhesion of the electrode to the base substrate is carefully controlled so that the electrode can be processed on the substrate and subsequently removed from the substrate after processing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application which claims the benefitunder 35 U.S.C. § 121 of U.S. patent application Ser. No. 15/892,162,filed Feb. 8, 2018, the contents of which are incorporated herein byreference.

TECHNICAL FIELD

The invention relates to electrodes for glucose sensors and methods offabricating the same.

BACKGROUND OF THE INVENTION

Electrochemical sensors are commonly used to detect or measure theconcentrations of in vivo analytes, such as glucose. Typically in suchanalyte sensing systems, an analyte (or a species derived from it) iselectro-active and generates a detectable signal at an electrode in thesensor. This signal is then correlated with the presence orconcentration of the analyte within a biological sample. In someconventional sensors, an enzyme is provided that reacts with the analyteto be measured, the byproduct of the reaction being qualified orquantified at the electrode. In one conventional glucose sensor,immobilized glucose oxidase catalyzes the oxidation of glucose to formhydrogen peroxide, which is then quantified by amperometric measurements(e.g. change in electrical current) through one or more electrodes.

A variety of electrochemical glucose sensors are multi-layered,comprising electrodes on top of and/or coated by layers of variousmaterials. Multilayered sensors have a number of desirable propertiesincluding the fact that the functional properties of such sensors can betailored by altering certain design parameters (e.g. number of internallayers, layer thickness, electrodes area and architecture etc). However,the inventors of the present invention have found that undesirableinteractions between the anode and cathode in conventional sensorsdegrade sensor performance. What is needed, then, are sensor fabricationmethods and electrode structures that reduce or prevent unwantedcathode-anode interactions, thereby improving sensor performance. Thepresent disclosure satisfies this need.

SUMMARY OF THE INVENTION

The present disclosure describes an analyte sensor apparatus (e.g.,glucose sensor), comprising a working electrode; a counter electrode;and an insulation layer between the working electrode and the counterelectrode, wherein the working electrode is spatially separated from thecounter electrode by a distance of at least 1 micrometer, the workingelectrode comprises a metal composition having an electroactive surface,and the working electrode and the counter electrode arenon-interdigitated. An analyte sensing layer on the working electrodedetectably alters electrical current at the working electrode in apresence of an analyte.

In one or more embodiments, the working electrode and the counterelectrode are on a same side of the analyte sensor apparatus.

In other embodiments, the working electrode is on a first or top side ofan insulation layer and the counter electrode is on a second or backside of the insulation layer opposite the first side, e.g., therebyplacing an electrode on both the top-side and back-side of the sensorflex. Conventional methods only place the electrode on the top-side ofthe sensor flex. Thus, embodiments of the present invention obviate theneed to have multiple sensor flexes in one device.

Illustrative embodiments further comprise a reference electrode on thefirst side of the insulation layer; insulation between the referenceelectrode and the working electrode; first metal electrically contactingthe working electrode, the first metal comprising a first contact pad;second metal electrically contacting the counter electrode, the secondmetal comprising a second contact pad. Example materials for theinsulation layer and the insulation include, but are not limited to,polyimide, and the working electrode, the counter electrode, theinsulation layer, the insulation, and the analyte sensing layer may beflexible.

Example electrode surface metals for the counter electrode include, butare not limited to, gold, platinum, silver, etc. In one or moreembodiments, the conventional electroplated platinum layer in theworking electrode is replaced by a layer including platinum pillars, andthe conventionally electroplated reference electrode is replaced by areference electrode including silver-silver chloride that is screenprinted or dispensed, etc.

In yet further embodiments, the counter electrode comprises physicalvapor deposited (PVD) metal removed from a rigid substrate, or theapparatus further comprises a base layer attached to the counterelectrode and physical vapor deposited metal on the base layer, and thephysical vapor deposited metal removed from a rigid substrate.

As illustrated herein, embodiments of the sensors disclosed hereinexhibit surprising and and unexpected performance improvements overconventional sensors. In one or more examples, a separation,configuration, and arrangement of the working electrode and the counterelectrode are such that, in response to a constant analyteconcentration, (1) the electrical current varies by less than 15% over aperiod of 31 days, and/or (2) chemical products created by reactions ateach of the working electrode and the counter electrode reactions do notinterfere or have detrimental interactions with performance of theworking electrode and the counter electrode.

The present disclosure further reports on techniques developed tocontrol adhesion of the PVD metal film through the PVD process. Avariety of PVD parameters were evaluated through multiple Design ofExperiments (DOEs). Pressure was unexpectedly and surprisinglydiscovered to have the largest and most significant impact on adhesion,and controlling and changing the pressure during the PVD the processachieved different levels of adhesion.

The present disclosure further reports on how the deposition of a roughor pillar like structure in the metal film, reducing surface areacontact to the substrate/surface in a highly controllable manner, aidsin controlling adhesion when the deposition pressure is modulated.

In one or more examples, the PVD process parameters include pressure ina range of 2-250 millitorr, PVD power in a range of 10 watts to 100kilowatts, and depositing metal having a thickness of at least 100Angstroms.

In one example, PVD deposition with pressure modulation is used tofabricate a Backside Counter Electrode (BCE) for a glucose sensor, wherethe metal to glass substrate adhesion is strong enough to surviveprocessing and laser cutting, but weak enough to allow easy physicalremoval from the glass substrate for assembly processes.

An illustrative fabrication method for an analyte sensor apparatuscomprises providing a base substrate; depositing metal on the basesubstrate using PVD; depositing a film comprising the insulation layer,the working electrode, and the counter electrode on the metal; defininganalyte sensors in the film; and removing the analyte sensors from thebase substrate. In one or more examples, the metal comprises a secondlayer on a first layer, the first layer between the second layer and theinsulation layer; the first layer is deposited at the pressurecomprising a first pressure, and the second layer is deposited at thepressure comprising a second pressure lower than the first pressure.

Another illustrative fabrication method for an analyte sensor apparatuscomprises depositing the insulation layer comprising a first polyimideinsulation layer on the metal; depositing and patterning second metal onthe first polyimide insulation layer; depositing a second insulationpolyimide layer on the first insulation polyimide layer and the secondmetal on the first insulation polyimide insulation layer; forming afirst opening and a second opening in the second insulation polyimidelayer; depositing third metal into the first opening, so as to form aworking electrode; depositing fourth metal into the second opening so asto form a reference electrode (RE); defining the analyte sensors in thefilm comprising the metal, the second metal, the third metal, the fourthmetal, the first insulation polyimide layer, the second insulationpolyimide layer, the working electrode, and the reference electrode; andremoving the analyte sensors from the base substrate, wherein the metalis the counter electrode.

Yet another illustrative fabrication method for an analyte sensorcomprises depositing a base layer comprising polyimide on the metal onthe base substrate; patterning a first opening in the base layer;depositing second metal in the first opening, forming a counterelectrode; depositing the insulating layer comprising a first polyimideinsulation layer on the base layer and the counter electrode; depositingand patterning third metal on the first polyimide insulation layer;depositing a second insulation polyimide layer on the first insulationpolyimide layer and the third metal on the first insulation polyimideinsulation layer; forming a second opening and a third opening in thesecond insulation polyimide layer; curing the base layer, the firstinsulation polyimide layer, and the second insulation polyimide layer;depositing fourth metal into the second opening, so as to form a workingelectrode; depositing fifth metal into the third opening so as to form areference electrode (RE); defining the analyte sensors in the filmcomprising the base polyimide layer, the first insulation polyimidelayer, the second insulation polyimide layer, and the electrodes; andremoving the analyte sensors from the base substrate.

In one or more embodiments, a set of at least 36 of the sensorsfabricated using the methods illustrated herein each have a workingelectrode separated from the counter electrode such that in response tothe same analyte concentration, the electrical currents outputted byeach of the sensors are within 15%.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription. It is to be understood, however, that the detaileddescription and specific examples, while indicating some embodiments ofthe present invention are given by way of illustration and notlimitation. Many changes and modifications within the scope of thepresent invention may be made without departing from the spirit thereof,and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D illustrate amperometric sensors with WE and CE on oppositesides, according to one or more embodiments.

FIG. 1E illustrates an amperometric sensor with WE and CE on a same sideof the device but separated by a distance D of at least 1 micrometerinches, according to one or more embodiments of the invention.

FIG. 1F and FIG. 1G compare structures of a control sensor (FIG. 1F)having an interdigitated working electrode and counter electrode, areference electrode, and wherein the working electrode, counterelectrode, and reference electrode are on one side only and aresufficiently close to exhibit undesirable electrode interactions, withsensors comprising electrodes on opposite sides (FIG. 1G) according toone or more embodiments of the present invention).

FIG. 1H illustrates the plurality of planar layered elements used in anamperometric sensor.

FIG. 2 provides a perspective view illustrating one type of subcutaneoussensor insertion set, a telemetered characteristic monitor transmitterdevice, and a data receiving device, elements that can be adapted foruse with embodiments of the invention.

FIG. 3 shows a schematic of a potentiostat that may be used to measurecurrent in embodiments of the present invention. As shown in FIG. 3 , apotentiostat 300 may include an op amp 310 that is connected in anelectrical circuit so as to have two inputs: Vset and Vmeasured. Asshown, Vmeasured is the measured value of the voltage between areference electrode and a working electrode. Vset, on the other hand, isthe optimally desired voltage across the working and referenceelectrodes. The current between the counter and reference electrode ismeasured, creating a current measurement (isig) that is output from thepotentiostat.

FIG. 4 illustrates an apparatus for depositing material usingsputtering, according to one or more embodiments of the presentinvention.

FIG. 5 illustrates a test sample comprising a layer stack on a glasssubstrate, according to one or more embodiments of the presentinvention.

FIG. 6A illustrates different patterns A-E of knife scratches or lasercutting marks applied to the layer stack on the glass substrate thatsimulate the types of marks and cuts that may be applied duringprocessing of an electrode in a glucose sensor according to one or moreembodiments of the present invention.

FIG. 6B illustrates the patterns applied to a silver layer on a glasssubstrate, showing that adhesion of the silver to the glass is too weakto allow reproduction of the markings on the silver layer.

FIGS. 7A-7D illustrate different adhesion scores assigned to samplesfabricated under different sputtering conditions according to one ormore embodiments of the present invention.

FIG. 8A illustrates the test sample without gold pillars and FIG. 8Billustrates the test sample with gold pillars, according to one or moreembodiments of the present invention.

FIG. 9 is a scanning electron microscope (SEM) image of the pillaredinterface between the glass substrate and the gold layer, according toone or more embodiments of the present invention.

FIGS. 10A-10D illustrate the film on the test samples fabricated usingvarious sputtering conditions and after laser cutting with an exampleelectrode pattern, according to one or more embodiments of the presentinvention.

FIG. 11 illustrates a Pareto chart of the standardized effects ofchanging pressure, power, and gold thickness on adhesion for the samplesfabricated using gold pillars at the interface between the gold layerand the glass substrate, according to one or more embodiments of thepresent invention.

FIG. 12 is a plot of the mean of rate as a function of pressure, power,and gold thickness, according to one or more embodiments of the presentinvention.

FIG. 13 is a contour plot of the rate versus thickness of the gold layerand pressure, according to one or more embodiments of the presentinvention.

FIG. 14 illustrates another test sample comprising a layer stack on aglass substrate, according to one or more embodiments of the presentinvention.

FIG. 15A illustrates the film on the test sample of FIG. 6A comprising agold layer deposited using sputtering conditions including 100 mTorrpressure, 1.5 kW power, for a duration of 5 minutes, according to one ormore embodiments of the present invention.

FIG. 15B illustrates the film on the test sample of FIG. 14 comprising afirst gold layer deposited using sputtering conditions including 100mTorr pressure, 1.5 kW power, for a duration of 5 minutes and the secondgold layer deposited using sputtering conditions including 4 mTorrpressure, 0.2 kW power, for a duration of 10 minutes, according to oneor more embodiments of the present invention.

FIGS. 15C, 15D, and 15E illustrate the film on the test sample of FIG.14 with two gold layers and deposited using the conditions for FIG. 15Bhas an adhesion that varies depending on position on the surface area.

FIG. 16 illustrates a Pareto chart of the standardized effects ofchanging pressure, power, and gold thickness on sputtering rate,according to one or more embodiments of the present invention.

FIG. 17 is a plot of the mean of the sputtering rate as a function ofpressure, power, and gold thickness, according to one or moreembodiments of the present invention.

FIG. 18 is a contour plot of the sputtering rate versus sputtering power(kW) and pressure (mTorr), according to one or more embodiments of thepresent invention.

FIG. 19 is a flowchart illustrating a method of fabricating a sensor orsensor flex according to one or more embodiments of the presentinvention.

FIG. 20 illustrates fabrication of the backside counter electrode sensorembodiment using the PVD methods described herein.

FIGS. 21A-21C illustrate SITS results for a control sensor (sensor 130illustrated in FIG. 1F), wherein FIGS. 21A and 21B plot electricalcurrent (ISIG) as a function of time (date in month of May) and FIG. 21Cplots Vcounter as a function of time (date in month of May), and thedifferent traces in FIGS. 21A-21C represent results for differentsensors.

FIGS. 21D-21F illustrates SITS results for the sensor of FIG. 1G(representing performance of the sensor FIG. 1D), according to one ormore embodiments of the present invention, wherein FIGS. 21D and 21Eplot ISIG as a function of time (date in month of May) and FIG. 21Fplots Vcounter (the voltage at the counter electrode) as a function oftime (date in month of May). The different traces in FIGS. 21D-21Frepresent results for different sensors and show capability of thesmooth backside CE design to support sensor function and importantlyreduced sensor-to-sensor performance variability and improvedperformance stability over the lifetime of the tests, for the sensor ofFIG. 1D as compared to the sensor of FIG. 1F.

FIG. 22 is a flowchart illustrating a method of fabricating a sensor orsensor flex according to one or more embodiments of the presentinvention.

FIG. 23 is a schematic illustrating a method of fabricating a sensor orsensor flex using the flowchart of FIG. 22 according to one or moreembodiments of the present invention.

FIG. 24 is a flowchart illustrating a method of depositing films on asubstrate, according to one or more embodiments of the presentinvention.

FIG. 25 is a flowchart illustrating a method of making a device on asubstrate, according to one or more embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise defined, all terms of art, notations, and otherscientific terms or terminology used herein are intended to have themeanings commonly understood by those of skill in the art to which thisinvention pertains. In some cases, terms with commonly understoodmeanings may be defined herein for clarity and/or for ready reference,and the inclusion of such definitions herein should not necessarily beconstrued to represent a substantial difference over what is generallyunderstood in the art. Many of the techniques and procedures describedor referenced herein are well understood and commonly employed usingconventional methodology by those skilled in the art.

All numbers recited in the specification and associated claims thatrefer to values that can be numerically characterized with a value otherthan a whole number (e.g. a thickness) are understood to be modified bythe term “about”. Where a range of values is provided, it is understoodthat each intervening value, to the tenth of the unit of the lower limitunless the context clearly dictates otherwise, between the upper andlower limit of that range and any other stated or intervening value inthat stated range, is encompassed within the invention. The upper andlower limits of these smaller ranges may independently be included inthe smaller ranges, and are also encompassed within the invention,subject to any specifically excluded limit in the stated range. Wherethe stated range includes one or both of the limits, ranges excludingeither or both of those included limits are also included in theinvention. Furthermore, all publications mentioned herein areincorporated herein by reference to disclose and describe the methodsand/or materials in connection with which the publications are cited.Publications cited herein are cited for their disclosure prior to thefiling date of the present application. Nothing here is to be construedas an admission that the inventors are not entitled to antedate thepublications by virtue of an earlier priority date or prior date ofinvention. Further the actual publication dates may be different fromthose shown and require independent verification.

As discussed in detail below, embodiments of the invention relate to theuse of an electrochemical sensor that measures a concentration of ananalyte of interest or a substance indicative of the concentration orpresence of the analyte in fluid. In some embodiments, the sensor is acontinuous device, for example a subcutaneous, transdermal, orintravascular device. In some embodiments, the device can analyze aplurality of intermittent blood samples. The sensor embodimentsdisclosed herein can use any known method, including invasive, minimallyinvasive, and non-invasive sensing techniques, to provide an outputsignal indicative of the concentration of the analyte of interest.Typically, the sensor is of the type that senses a product or reactantof an enzymatic reaction between an analyte and an enzyme in thepresence of oxygen as a measure of the analyte in vivo or in vitro. Suchsensors typically comprise a membrane surrounding the enzyme throughwhich an analyte migrates. The product is then measured usingelectrochemical methods and thus the output of an electrode systemfunctions as a measure of the analyte.

Embodiments of the invention disclosed herein provide sensors of thetype used, for example, in subcutaneous or transcutaneous monitoring ofblood glucose levels in a diabetic patient. A variety of implantable,electrochemical biosensors have been developed for the treatment ofdiabetes and other life-threatening diseases. Many existing sensordesigns use some form of immobilized enzyme to achieve theirbio-specificity. Embodiments of the invention described herein can beadapted and implemented with a wide variety of known electrochemicalsensors elements, including for example, those disclosed in U.S. PatentApplication Nos. 20050115832, 20050008671, 20070227907, 20400025238,20110319734, 20110152654 and Ser. No. 13/707,400 filed Dec. 6, 2012,U.S. Pat. Nos. 6,001,067, 6,702,857, 6,212,416, 6,119,028, 6,400,974,6,595,919, 6,141,573, 6,122,536, 6,512,939 5,605,152, 4,431,004,4,703,756, 6,514,718, 5,985,129, 5,390,691, 5,391, 250, 5,482,473,5,299,571, 5,568,806, 5,494,562, 6,120,676, 6,542,765, 7,033,336 as wellas PCT International Publication Numbers WO 01/58348, WO 04/021877, WO03/034902, WO 03/035117, WO 03/035891, WO 03/023388, WO 03/022128, WO03/022352, WO 03/023708, WO 03/036255, WO03/036310 WO 08/042,625, and WO03/074107, and European Patent Application EP 1153571, the contents ofeach of which are incorporated herein by reference.

A. Illustrative Embodiments of the Invention and AssociatedCharacteristics

Controllable adhesion of Physical Vapor Deposited (PVD) metal films is awide spread challenge and problem throughout the MEMS and semiconductorindustries as well as for flex circuit applications. For a variety ofapplications, Metal films often need to maintain very specific levels ofadhesion to surfaces/substrates they are deposited on. In some casesstrong adhesion is required, while in other applications weak adhesionis required. In the most challenging cases, a mixture of weak and strongadhesion is required, where the adhesion force is strong enough towithstand specific aspects of an application but weak enough for otheraspects of the application to function properly.

As illustrated herein, the present disclosure describes an efficientmethod to adjust and control the adhesion of PVD films deposited onsurfaces/substrates. A comprehensive series of studies evaluating PVDdeposition factors and their influence on adhesion property wereperformed and pressure was discovered to be a critically significantfactor for adjusting adhesion. This single factor is a key component ofPVD deposition and is controllable in the PVD process; as such, pressureis an ideal factor to utilize for controlling film adhesion.Illustrative methods described herein are applicable to all PVD systemsutilized for depositing thin or thick films.

Of particular interest from a device perspective, controlling adhesionusing pressure modulation enables fabrication and manufacturing ofdevices where PVD layers are deposited in direct contact with a carriersubstrate and while being releasable based on an adhesion metric. FIG.1A illustrates an example of such as a device useful in diabetesapplications such as, but not limited to, continuous glucose monitoring(CGM) sensors where electrodes are on both sides (topside and backside)of a single sensor flex. As illustrated herein, the pressure modulationprovides a highly efficient approach to adjust the adhesion of thebackside electrode to the carrier substrate, enabling release at aspecific point in the overall downstream manufacturing process. Placingthe contact pads for each of the electrodes on either side of the sensorflex enables a wider array of connection schemes to the transmitter.Moreover, adjusting adhesion can be used to minimize the amount ofnewly-added processing steps for the backside electrode. As a whole, theadhesion control illustrated herein may be used to reduce manufacturingcomplexity, compared to conventional sensors, by significant margins.

Importantly, the novel methods of controlling adhesion described hereincan be accomplished using standard materials, equipment and facilitiesassociated with PVD.

The methods for forming analyte sensors that comprise the electrodesdisclosed herein can include a number of steps. For example, suchmethods can include forming a working electrode, a counter electrode anda reference electrode on the base substrate and/or forming a pluralityof contact pads on the base substrate, and/or forming a plurality ofelectrical conduits on the base substrate. In certain embodiments of theinvention, the methods comprise forming a plurality of workingelectrodes, counter electrodes and reference electrodes clusteredtogether in units consisting essentially of one working electrode, onecounter electrode and one reference electrode. The electrodes are formedon the base substrate and these clustered units are longitudinallydistributed on at least one longitudinal arm of the base substrate in arepeating pattern of units. Optionally in such methods, the workingelectrode is formed as an array of electrically conductive membersdisposed on the base substrate, the electrically conductive members arecircular and have a diameter between 10 μm and 400 μm, and the arraycomprises at least 10 electrically conductive members. The methods canfurther comprise forming an analyte sensing layer on the workingelectrode, wherein the analyte sensing layer detectably alters theelectrical current at the working electrode in the presence of ananalyte. Typically these methods also include forming an analytemodulating layer on the analyte sensing layer, wherein the analytemodulating layer modulates the diffusion of analyte therethrough.

Yet another embodiment of the invention is an analyte sensor apparatusthat includes a base substrate comprising a well that holds a metalelectrode composition formed using the sputtering processes disclosedherein. In such embodiments, the structure of the platinum compositionis formed to include a central planar region and an edge or ridge likeregion that surrounds the central planar region. In such embodiments,the thickness or height of the metal composition at the edge is lessthan 2× the average thickness of metal composition in the central planarregion. In certain embodiments of the invention, the well comprises alip that surrounds the well; and the edge region of the metalcomposition is below the lip of the well. Typically in theseembodiments, both the central planar region forms an electroactivesurface of a working electrode in the sensor. Sensor embodiments of theinvention typically include additional layers of material coated overthe working electrode, for example an analyte sensing layer disposedover the working electrode, one that detectably alters the electricalcurrent at the working electrode in the presence of an analyte as wellas an analyte modulating layer disposed over the analyte sensing layerthat modulates the diffusion of analyte therethrough.

In typical embodiments of the invention, the electrode is formed in awell of a base substrate comprising a dielectric material (e.g. apolyimide). Typically, the well includes a conductive material disposedat the bottom of the well (e.g. Au). Optionally the well in the basesubstrate is rectangular or circular. In certain embodiments of theinvention, the base substrate comprises at least 10, 20 or 30 wellsformed into a microarray. In typical sensor embodiments, a basesubstrate is formed so that it includes a well that comprises a lipsurrounding the well. In certain processes disclosed herein, the metalcomposition is sputtered so that the metal composition is below the lipof the well. In addition, a variety of different electrically conductiveelements can be disposed on the base substrate. In some embodiments ofthe invention, the base substrate comprises a plurality of referenceelectrodes, a plurality of working electrodes and a plurality of counterelectrodes clustered together in units consisting essentially of oneworking electrode, one counter electrode and one reference electrode,and the clustered units are longitudinally distributed on the basesubstrate in a repeating pattern of units.

Embodiments of the invention include further elements designed for usewith the sensor apparatuses that are disclosed herein, for example thosethat are designed to analyze electrical signal data obtained fromsputtered electrodes disposed on the base substrate. In some embodimentsof the invention, the analyte sensor apparatus includes a processor anda computer-readable program code having instructions, which whenexecuted, cause the processor to assess electrochemical signal dataobtained from at least one working electrode and then compute analyteconcentrations based upon the electrochemical signal data obtained fromthe working electrode. In certain embodiments of the invention, theprocessor compares electrochemical signal data obtained from multipleworking electrodes in order to, for example, adapt different electrodesto sense different analytes, and/or to focus on different concentrationranges of a single analyte; and/or to identify or characterize spurioussensor signals (e.g. sensor noise, signals caused by interferingcompounds and the like) so as to enhance the accuracy of the sensorreadings.

In some embodiments of the invention, the base structure comprises aflexible yet rigid and flat structure suitable for use inphotolithographic mask and etch processes. In this regard, the basestructure typically includes at least one surface having a high degreeof uniform flatness. Base structure materials can include, for example,metals such as stainless steel, aluminum and nickel titanium memoryalloys (e.g. NITINOL) as well as polymeric/plastic materials such asdelrin, etc. Base structure materials can be made from, or coated with,a dielectric material. In some embodiments, the base structure isnon-rigid and can be a layer of film or insulation that is used as asubstrate for patterning electrical elements (e.g. electrodes, tracesand the like), for example plastics such as polyimides and the like. Aninitial step in the methods of the invention typically includes theformation of a base substrate of the sensor. Optionally, the planarsheet of material is formed and/or disposed on a support such as a glassor ceramic plate during sensor production. The base structure can bedisposed on a support (e.g. a glass plate) by PVD. This can then befollowed by a sequence of photolithographic and/or chemical mask andetch steps to form the electrically conductive components. In anillustrative form, the base substrate comprises a thin film sheet ofinsulative material, such as a polyimide substrate that is used topattern electrical elements. The base substrate structure may compriseone or more of a variety of elements including, but not limited to,carbon, nitrogen, oxygen, silicon, sapphire, diamond, aluminum, copper,gallium, arsenic, lanthanum, neodymium, strontium, titanium, yttrium, orcombinations thereof.

The methods of the invention include forming an electrically conductivelayer on the base substrate that function as one or more sensingelements. Typically these sensing elements include electrodes,electrical conduits (e.g. traces and the like), contact pads and thelike that are formed by one of the variety of methods known in the artsuch as photolithography, etching and rinsing to define the geometry ofthe active electrodes. The electrodes can then be made fromelectrochemically active materials having defined architectures, forexample by using sputtered Pt black for the working electrode. A sensorlayer such as a analyte sensing enzyme layer can then be disposed on thesensing layer by electrochemical deposition or a method other thanelectrochemical deposition such as spin coating, followed by vaporcrosslinking, for example with a dialdehyde (glutaraldehyde) or acarbodi-imide.

In an exemplary embodiment of the invention, the base substrate isinitially coated with a thin film conductive layer by electrodedeposition, surface sputtering, or other suitable patterning or otherprocess step. In one embodiment this conductive layer may be provided asa plurality of thin film conductive layers, such as an initialchrome-based layer suitable for chemical adhesion to a polyimide basesubstrate followed by subsequent formation of thin film gold-based andchrome-based layers in sequence. In alternative embodiments, otherelectrode layer conformations or materials can be used. The conductivelayer is then covered, in accordance with conventional photolithographictechniques, with a selected photoresist coating, and a contact mask canbe applied over the photoresist coating for suitable photoimaging. Thecontact mask typically includes one or more conductor trace patterns forappropriate exposure of the photoresist coating, followed by an etchstep resulting in a plurality of conductive sensor traces remaining onthe base substrate. In an illustrative sensor construction designed foruse as a subcutaneous glucose sensor, each sensor trace can include twoor three parallel sensor elements corresponding with two or threeseparate electrodes such as a working electrode, a counter electrode anda reference electrode.

Embodiments of the invention include methods of adding a plurality ofmaterials to the surface(s) of the sputtered electrode(s). One suchembodiment of the invention is a method of making a sensor apparatus(e.g. a glucose sensor) for implantation within a mammal comprising thesteps of: providing a base substrate; forming a conductive layer on thebase substrate, wherein the conductive layer includes an electrodeformed from a sputtering process that generates metallic columns of acertain architecture, forming an analyte sensing layer on the conductivelayer, wherein the analyte sensing layer includes a composition that canalter the electrical current at the electrode in the conductive layer inthe presence of an analyte (e.g. glucose oxidase); optionally forming aprotein layer over the analyte sensing layer; forming an adhesionpromoting layer on the analyte sensing layer or the optional proteinlayer; forming an analyte modulating layer disposed on the adhesionpromoting layer, wherein the analyte modulating layer includes acomposition that modulates the diffusion of the analyte therethrough;and forming a cover layer disposed on at least a portion of the analytemodulating layer, wherein the cover layer further includes an apertureover at least a portion of the analyte modulating layer.

In the working embodiments of the invention that are disclosed herein,the analyte sensing layer comprises glucose oxidase. Optionally, theapparatus comprises an adhesion promoting layer disposed between theanalyte sensing layer and the analyte modulating layer. In someembodiments of the invention, the analyte modulating layer comprises ahydrophilic comb-copolymer having a central chain and a plurality ofside chains coupled to the central chain, wherein at least one sidechain comprises a silicone moiety. Typically, the apparatus comprises abiocompatible material on an external surface that is adapted to contactbiological tissues or fluids when implanted in vivo. In the workingembodiments of the invention that are disclosed herein, the analytesensor apparatus is an amperometric glucose sensor exhibits a highlydesirable oxygen response profile. In such embodiments, the amperometricglucose sensor generates a first signal in a solution comprising 100mg/dL glucose and 5% oxygen and a second signal in a solution comprising100 mg/dL glucose and 0.1% oxygen (i.e. test conditions where the onlysubstantive difference is the % oxygen), and the first signal and thesecond signal differ by less than 10%.

Additional functional coatings or cover layers can then be applied to anelectrode or other senor element by any one of a wide variety of methodsknown in the art, such as spraying, dipping, etc. Some embodiments ofthe present invention include an analyte modulating layer deposited overan enzyme-containing layer that is disposed over a working electrode. Inaddition to its use in modulating the amount of analyte(s) that contactsthe active sensor surface, by utilizing an analyte limiting membranelayer, the problem of sensor fouling by extraneous materials is alsoobviated. As is known in the art, the thickness of the analytemodulating membrane layer can influence the amount of analyte thatreaches the active enzyme. Consequently, its application is typicallycarried out under defined processing conditions, and its dimensionalthickness is closely controlled. Microfabrication of the underlyinglayers can be a factor which affects dimensional control over theanalyte modulating membrane layer as well as the exact composition ofthe analyte limiting membrane layer material itself. In this regard, ithas been discovered that several types of copolymers, for example, acopolymer of a siloxane and a nonsiloxane moiety, are particularlyuseful. These materials can be microdispensed or spin-coated to acontrolled thickness. Their final architecture may also be designed bypatterning and photolithographic techniques in conformity with the otherdiscrete structures described herein.

In some embodiments of the invention, the sensor is made by methodswhich apply an analyte modulating layer that comprises a hydrophilicmembrane coating which can regulate the amount of analyte that cancontact the enzyme of the sensor layer. For example, a cover layer thatis added to the glucose sensing elements of the invention can comprise aglucose limiting membrane, which regulates the amount of glucose thatcontacts glucose oxidase enzyme layer on an electrode. Such glucoselimiting membranes can be made from a wide variety of materials known tobe suitable for such purposes, e.g., silicones such as polydimethylsiloxane and the like, polyurethanes, cellulose acetates, Nafion,polyester sulfonic acids (e.g. Kodak AQ), hydrogels or any othermembrane known to those skilled in the art that is suitable for suchpurposes. In certain embodiments of the invention, the analytemodulating layer comprises a hydrophilic polymer. In some embodiments ofthe invention, the analyte modulating layer comprises a linearpolyurethane/polyurea polymer and/or a branched acrylate polymer; and/ora mixture of such polymers.

In some embodiments of the methods of invention, an adhesion promoterlayer is disposed between a cover layer (e.g. an analyte modulatingmembrane layer) and an analyte sensing layer in order to facilitatetheir contact and is selected for its ability to increase the stabilityof the sensor apparatus. As noted herein, compositions of the adhesionpromoter layer are selected to provide a number of desirablecharacteristics in addition to an ability to provide sensor stability.For example, some compositions for use in the adhesion promoter layerare selected to play a role in interference rejection as well as tocontrol mass transfer of the desired analyte. The adhesion promoterlayer can be made from any one of a wide variety of materials known inthe art to facilitate the bonding between such layers and can be appliedby any one of a wide variety of methods known in the art.

The finished sensors produced by such processes are typically quicklyand easily removed from a support structure (if one is used), forexample, by cutting along a line surrounding each sensor on the supportstructure and then peeling from the support structure. The cutting stepcan use methods typically used in this art such as those that include aUV laser cutting device that is used to cut through the base and coverlayers and the functional coating layers along a line surrounding orcircumscribing each sensor, typically in at least slight outward spacedrelation from the conductive elements so that the sufficientinterconnected base and cover layer material remains to seal the sideedges of the finished sensor. As illustrated herein, since the basesubstrate is sufficiently weakly adhered directly to the underlyingsupport, the sensors can be lifted quickly and easily peeled from thesupport structure, without significant further processing steps orpotential damage due to stresses incurred by excessive force beingapplied to peel the attached sensors from the support structure. Thesupport structure can thereafter be cleaned and reused, or otherwisediscarded. The functional coating layer(s) can be applied either beforeor after other sensor components are removed from the support structure(e.g. by cutting).

Embodiments of the invention also include methods of sensing an analyte(e.g. glucose) within the body of a mammal (e.g. a diabetic patient),the method comprising implanting a analyte sensor embodiment disclosedherein into an in vivo environment and then sensing one or moreelectrical fluctuations such as alteration in current at the workingelectrode and correlating the alteration in current with the presence ofthe analyte, so that the analyte is sensed. Typically, this methodcomprises implanting a glucose sensor disclosed herein within theinterstitial space of a diabetic individual, sensing an alteration incurrent at the working electrode in the presence of glucose; and thencorrelating the alteration in current with the presence of the glucose,so that glucose is sensed. While typical embodiments of the inventionpertain to glucose sensors, the sputtered sensor electrodes disclosedherein can be adapted for use with a wide variety of devices known inthe art.

As discussed in detail below, embodiments of the invention includesensor systems comprising addition elements designed to facilitatesensing of an analyte. For example, in certain embodiments of theinvention, the base material comprising the sensor electrodes isdisposed within a housing (e.g. a lumen of a catheter) and/or associatedwith other components that facilitate analyte (e.g. glucose) sensing.One illustrative sensor system comprises a processor, a base comprisinga first longitudinal member and a second longitudinal member, the firstand second longitudinal members each comprising at least one electrodehaving an electrochemically reactive surface, wherein theelectrochemically reactive surface generates an electrochemical signalthat is assessed by the processor in the presence of an analyte; and acomputer-readable program code having instructions, which when executedcause the processor to assess electrochemical signal data obtained fromthe electrodes; and compute an analyte presence or concentration basedupon the electrochemical signal data obtained from the electrode.

Embodiments of the invention described herein can also be adapted andimplemented with amperometric sensor structures, for example thosedisclosed in U.S. Patent Application Publication Nos. 20070227907,20400025238, 20110319734 and 20110152654, the contents of each of whichare incorporated herein by reference.

B. Illustrative Analyte Sensor Constituents and Sensor Stacks Used inEmbodiments of the Invention

The following disclosure provides examples of typicalelements/constituents used in sensor embodiments of the invention. Whilethese elements can be described as discrete units (e.g. layers), thoseof skill in the art understand that sensors can be designed to containelements having a combination of some or all of the material propertiesand/or functions of the elements/constituents discussed below (e.g. anelement that serves both as a supporting base constituent and/or aconductive constituent and/or a matrix for the analyte sensingconstituent and which further functions as an electrode in the sensor).Those in the art understand that these thin film analyte sensors can beadapted for use in a number of sensor systems such as those describedbelow.

FIGS. 1A-1D illustrate embodiments of an analyte sensor apparatus 100a-100 d comprising a working electrode (WE) on a first side 102 a of aninsulation layer 104 a, 104 b and a counter electrode (CE or BCE) on asecond side 102 b of the insulation layer 104 a, 104 b so that theinsulation layer 104 a, 104 b is between the CE/BCE and the WE. FIGS.1A-1D further illustrate a reference electrode (RE) on the first side102 a of the insulation layer 104 a, 104 b and insulation 106 betweenthe RE and WE. Metal 108 deposited on the insulation layer 104 a, 104 belectrically contacts the WE and comprises a contact pad 110 forcontacting the WE. Metal 112 or CE on the insulation layer 104 acomprises the CE and contact pad 114 for contacting the CE. Also shownin FIGS. 1A-1C is a base layer 116 on the CE and second insulation layer118 on the first insulation layer 104 a, 104 b and metal 108.

The WE comprises a metal composition 120 having an electroactive surface122. In the examples illustrated in FIGS. 1A-1D, the WE comprisespillars 124 including the metal composition 120 and having theelectroactive surface 122.

FIG. 1D illustrates a sensor 100 d embodiment wherein the backside CEis/comprises a layer capable of controlling adhesion to a substrate aswell as an electrode in the sensor apparatus 100 d.

FIG. 1E illustrates an analyte sensor apparatus 100 e, comprising aworking electrode WE and a CE on a first side (same side) 126 of asubstrate 128 and wherein the WE and the CE are spatially separated by adistance D of at least 1 micrometer inches, e.g., in a range of 1micrometer-20 micrometers. The WE and the CE are non-interdigitated. Thedistance D is sufficiently large to reduce unwanted interactions betweenthe WE and the CE (i.e., reducing the impact of the oxidation reactionat one electrode on the reduction reaction at the other electrode, andvice versa).

In one or more embodiments, the devices of FIG. 1A-1E are fabricatedusing PVD and/or electroplating.

FIGS. 1F and 1G compare the structure of a control sensor 130 having aninterdigitated working electrode 132 and counter electrode 134 and areference electrode 136 on one side 126 of the device 130 with thesensor FIG. 1G representing embodiments 100 a-100 d comprising electrodeWE on a first side 102 a and electrode CE on a second side 102 b. Theworking electrode 132 and control electrode 134 in the control device130 also have a smaller separation that causes undesirable interactionsbetween the electrodes 132 and 134.

In one or more embodiments, the sensors 100 a-e includes furtherlayers/coatings/constituents (e.g., on the WE) so as to enable operationas a glucose sensor (e.g., for diabetes applications), as illustrated inFIG. 1H. The further constituents include the following.

Base Constituent

Sensors of the invention typically include a base constituent (see, e.g.element 104 b in FIG. 1D, element 402 in FIG. 1H, element 128 in FIG.1E, or element 116 in FIGS. 1A-1D). The term “base constituent” is usedherein according to art accepted terminology and refers to theconstituent in the apparatus that typically provides a supporting matrixfor the plurality of constituents that are stacked on top of one anotherand comprise the functioning sensor. In one form, the base constituentcomprises a thin film sheet of insulative (e.g. electrically insulativeand/or water impermeable) material. This base constituent can be made ofa wide variety of materials having desirable qualities such asdielectric properties, water impermeability and hermeticity. Somematerials include metallic, and/or ceramic and/or polymeric substratesor the like.

Conductive Constituent

The electrochemical sensors of the invention typically include aconductive constituent disposed upon the base constituent that includesat least one electrode comprising a metal for contacting an analyte orits byproduct (e.g. oxygen and/or hydrogen peroxide) to be assayed (see,e.g. WE in FIGS. 1B-1F). The term “conductive constituent” is usedherein according to art accepted terminology and refers to electricallyconductive sensor elements such as electrodes, contact pads, traces andthe like. An illustrative example of this is a conductive constituentthat forms a working electrode that can measure an increase or decreasein current in response to exposure to a stimuli such as the change inthe concentration of an analyte or its byproduct as compared to areference electrode that does not experience the change in theconcentration of the analyte, a coreactant (e.g. oxygen) used when theanalyte interacts with a composition (e.g. the enzyme glucose oxidase)present in analyte sensing constituent 410 or a reaction product of thisinteraction (e.g. hydrogen peroxide). Illustrative examples of suchelements include electrodes which are capable of producing variabledetectable signals in the presence of variable concentrations ofmolecules such as hydrogen peroxide or oxygen.

In addition to the working electrode, the analyte sensors of theinvention typically include a reference electrode (RE) or a combinedreference and counter electrode (also termed a quasi-reference electrodeor a counter/reference electrode). If the sensor does not have acounter/reference electrode then it may include a separate counterelectrode (CE), which may be made from the same or different materialsas the working electrode. Typical sensors of the present invention haveone or more working electrodes and one or more counter, reference,and/or counter/reference electrodes. One embodiment of the sensor of thepresent invention has two, three or four or more working electrodes.These working electrodes in the sensor may be integrally connected orthey may be kept separate. Optionally, the electrodes can be disposed ona single surface or side of the sensor structure. Alternatively, theelectrodes can be disposed on a multiple surfaces or sides of the sensorstructure. In certain embodiments of the invention, the reactivesurfaces of the electrodes are of different relative areas/sizes, forexample a 1× reference electrode, a 3.2× working electrode and a 6.3×counter electrode.

Interference Rejection Constituent

The electrochemical sensors of the invention optionally include aninterference rejection constituent disposed between the surface of theelectrode and the environment to be assayed. In particular, certainsensor embodiments rely on the oxidation and/or reduction of hydrogenperoxide generated by enzymatic reactions on the surface of a workingelectrode at a constant applied potential. Because amperometricdetection based on direct oxidation of hydrogen peroxide requires arelatively high oxidation potential, sensors employing this detectionscheme may suffer interference from oxidizable species that are presentin biological fluids such as ascorbic acid, uric acid and acetaminophen.In this context, the term “interference rejection constituent” is usedherein according to art accepted terminology and refers to a coating ormembrane in the sensor that functions to inhibit spurious signalsgenerated by such oxidizable species which interfere with the detectionof the signal generated by the analyte to be sensed. Certaininterference rejection constituents function via size exclusion (e.g. byexcluding interfering species of a specific size). Examples ofinterference rejection constituents include one or more layers orcoatings of compounds such as hydrophilic polyurethanes, celluloseacetate (including cellulose acetate incorporating agents such aspoly(ethylene glycol), polyethersulfones, polytetra-fluoroethylenes, theperfluoronated ionomer Nafion™, polyphenylenediamine, epoxy and thelike.

Analyte Sensing Constituent

The electrochemical sensors of the invention include an analyte sensingconstituent disposed on the electrodes of the sensor (see, e.g. element410 in FIG. 1H). The term “analyte sensing constituent” is used hereinaccording to art accepted terminology and refers to a constituentcomprising a material that is capable of recognizing or reacting with ananalyte whose presence is to be detected by the analyte sensorapparatus. Typically, this material in the analyte sensing constituentproduces a detectable signal after interacting with the analyte to besensed, typically via the electrodes of the conductive constituent. Inthis regard, the analyte sensing constituent and the electrodes of theconductive constituent work in combination to produce the electricalsignal that is read by an apparatus associated with the analyte sensor.Typically, the analyte sensing constituent comprises an oxidoreductaseenzyme capable of reacting with and/or producing a molecule whose changein concentration can be measured by measuring the change in the currentat an electrode of the conductive constituent (e.g. oxygen and/orhydrogen peroxide), for example the enzyme glucose oxidase. An enzymecapable of producing a molecule such as hydrogen peroxide can bedisposed on the electrodes according to a number of processes known inthe art. The analyte sensing constituent can coat all or a portion ofthe various electrodes of the sensor. In this context, the analytesensing constituent may coat the electrodes to an equivalent degree.Alternatively, the analyte sensing constituent may coat differentelectrodes to different degrees, with for example the coated surface ofthe working electrode being larger than the coated surface of thecounter and/or reference electrode.

Typical sensor embodiments of this element of the invention utilize anenzyme (e.g. glucose oxidase) that has been combined with a secondprotein (e.g. albumin) in a fixed ratio (e.g. one that is typicallyoptimized for glucose oxidase stabilizing properties) and then appliedon the surface of an electrode to form a thin enzyme constituent. In atypical embodiment, the analyte sensing constituent comprises a GOx andHSA mixture. In a typical embodiment of an analyte sensing constituenthaving GOx, the GOx reacts with glucose present in the sensingenvironment (e.g. the body of a mammal) and generates hydrogen peroxide.

As noted above, the enzyme and the second protein (e.g. an albumin) aretypically treated to form a crosslinked matrix (e.g. by adding across-linking agent to the protein mixture). As is known in the art,crosslinking conditions may be manipulated to modulate factors such asthe retained biological activity of the enzyme, its mechanical and/oroperational stability. Illustrative crosslinking procedures aredescribed in U.S. patent application Ser. No. 10/335,506 and PCTpublication WO 03/035891 which are incorporated herein by reference. Forexample, an amine cross-linking reagent, such as, but not limited to,glutaraldehyde, can be added to the protein mixture. The addition of across-linking reagent to the protein mixture creates a protein paste.The concentration of the cross-linking reagent to be added may varyaccording to the concentration of the protein mixture. Whileglutaraldehyde is an illustrative crosslinking reagent, othercross-linking reagents may also be used or may be used in place ofglutaraldehyde. Other suitable cross-linkers also may be used, as willbe evident to those skilled in the art.

As noted above, in some embodiments of the invention, the analytesensing constituent includes an agent (e.g. glucose oxidase) capable ofproducing a signal (e.g. a change in oxygen and/or hydrogen peroxideconcentrations) that can be sensed by the electrically conductiveelements (e.g. electrodes which sense changes in oxygen and/or hydrogenperoxide concentrations). However, other useful analyte sensingconstituents can be formed from any composition that is capable ofproducing a detectable signal that can be sensed by the electricallyconductive elements after interacting with a target analyte whosepresence is to be detected. In some embodiments, the compositioncomprises an enzyme that modulates hydrogen peroxide concentrations uponreaction with an analyte to be sensed. Alternatively, the compositioncomprises an enzyme that modulates oxygen concentrations upon reactionwith an analyte to be sensed. In this context, a wide variety of enzymesthat either use or produce hydrogen peroxide and/or oxygen in a reactionwith a physiological analyte are known in the art and these enzymes canbe readily incorporated into the analyte sensing constituentcomposition. A variety of other enzymes known in the art can produceand/or utilize compounds whose modulation can be detected byelectrically conductive elements such as the electrodes that areincorporated into the sensor designs described herein. Such enzymesinclude for example, enzymes specifically described in Table 1, pages15-29 and/or Table 18, pages 111-112 of Protein Immobilization:Fundamentals and Applications (Bioprocess Technology, Vol 14) by RichardF. Taylor (Editor) Publisher: Marcel Dekker; Jan. 7, 1991) the entirecontents of which are incorporated herein by reference.

Protein Constituent

The electrochemical sensors of the invention optionally include aprotein constituent disposed between the analyte sensing constituent andthe analyte modulating constituent (see, e.g. element 416 in FIG. 1H).The term “protein constituent” is used herein according to art acceptedterminology and refers to constituent containing a carrier protein orthe like that is selected for compatibility with the analyte sensingconstituent and/or the analyte modulating constituent. In typicalembodiments, the protein constituent comprises an albumin such as humanserum albumin. The HSA concentration may vary between about 0.5%-30%(w/v). Typically the HSA concentration is about 1-10% w/v, and mosttypically is about 5% w/v. In alternative embodiments of the invention,collagen or BSA or other structural proteins used in these contexts canbe used instead of or in addition to HSA. This constituent is typicallycrosslinked on the analyte sensing constituent according to art acceptedprotocols.

Adhesion Promoting Constituent

The electrochemical sensors of the invention can include one or moreadhesion promoting (AP) constituents (see, e.g. element 414 in FIG. 1H).The term “adhesion promoting constituent” is used herein according toart accepted terminology and refers to a constituent that includesmaterials selected for their ability to promote adhesion betweenadjoining constituents in the sensor. Typically, the adhesion promotingconstituent is disposed between the analyte sensing constituent and theanalyte modulating constituent. Typically, the adhesion promotingconstituent is disposed between the optional protein constituent and theanalyte modulating constituent. The adhesion promoter constituent can bemade from any one of a wide variety of materials known in the art tofacilitate the bonding between such constituents and can be applied byany one of a wide variety of methods known in the art. Typically, theadhesion promoter constituent comprises a silane compound such as3-aminopropyltrimethoxysilane.

Analyte Modulating Constituent

The electrochemical sensors of the invention include an analytemodulating constituent disposed on the sensor (see, e.g. element 412 inFIG. 1H). The term “analyte modulating constituent” is used hereinaccording to art accepted terminology and refers to a constituent thattypically forms a membrane on the sensor that operates to modulate thediffusion of one or more analytes, such as glucose, through theconstituent. In certain embodiments of the invention, the analytemodulating constituent is an analyte-limiting membrane which operates toprevent or restrict the diffusion of one or more analytes, such asglucose, through the constituents. In other embodiments of theinvention, the analyte-modulating constituent operates to facilitate thediffusion of one or more analytes, through the constituents. Optionally,such analyte modulating constituents can be formed to prevent orrestrict the diffusion of one type of molecule through the constituent(e.g. glucose), while at the same time allowing or even facilitating thediffusion of other types of molecules through the constituent (e.g. O₂).

With respect to glucose sensors, in known enzyme electrodes, glucose andoxygen from blood, as well as some interferants, such as ascorbic acidand uric acid, diffuse through a primary membrane of the sensor. As theglucose, oxygen and interferants reach the analyte sensing constituent,an enzyme, such as glucose oxidase, catalyzes the conversion of glucoseto hydrogen peroxide and gluconolactone. The hydrogen peroxide maydiffuse back through the analyte modulating constituent, or it maydiffuse to an electrode where it can be reacted to form oxygen and aproton to produce a current that is proportional to the glucoseconcentration. The analyte modulating sensor membrane assembly servesseveral functions, including selectively allowing the passage of glucosetherethrough (see, e.g. U.S. Patent Application No. 2011-0152654).

Cover Constituent

The electrochemical sensors of the invention include one or more coverconstituents, which are typically electrically insulating protectiveconstituents (see, e.g. element 406 in FIG. 1H). Typically, such coverconstituents can be in the form of a coating, sheath or tube and aredisposed on at least a portion of the analyte modulating constituent.Acceptable polymer coatings for use as the insulating protective coverconstituent can include, but are not limited to, non-toxic biocompatiblepolymers such as silicone compounds, polyimides, biocompatible soldermasks, epoxy acrylate copolymers, or the like. Further, these coatingscan be photo-imageable to facilitate photolithographic forming ofapertures through to the conductive constituent. A typical coverconstituent comprises spun on silicone. As is known in the art, thisconstituent can be a commercially available RTV (room temperaturevulcanized) silicone composition. A typical chemistry in this context ispolydimethyl siloxane (acetoxy based).

FIG. 1H illustrates a cross-section of a typical sensor embodiment 400of the present invention that includes constituents discussed above.This sensor embodiment is formed from a plurality of components that aretypically in the form of layers of various conductive and non-conductiveconstituents disposed on each other according to art accepted methodsand/or the specific methods of the invention disclosed herein. Thecomponents of the sensor are typically characterized herein as layersbecause, for example, it allows for a facile characterization of thesensor structure shown in FIG. 1H. Artisans will understand however,that in certain embodiments of the invention, the sensor constituentsare combined such that multiple constituents form one or moreheterogeneous layers. In this context, those of skill in the artunderstand that the ordering of the layered constituents can be alteredin various embodiments of the invention.

The embodiment shown in FIG. 1H includes a base substrate layer 402 tosupport the sensor 400. The base substrate layer 402 can be made of amaterial such as a metal and/or a ceramic and/or a polymeric substrate,which may be self-supporting or further supported by another material asis known in the art. Embodiments of the invention include a conductivelayer 404 which is disposed on and/or combined with the base substratelayer 402. Typically, the conductive layer 404 comprises one or moreelectrically conductive elements that function as electrodes. Anoperating sensor 400 typically includes a plurality of electrodes suchas a working electrode, a counter electrode and a reference electrode.Other embodiments may also include a plurality of working and/or counterand/or reference electrodes and/or one or more electrodes that performsmultiple functions, for example one that functions as both as areference and a counter electrode.

As discussed in detail below, the base layer 402 and/or conductive layer404 can be generated using many known techniques and materials. Incertain embodiments of the invention, the electrical circuit of thesensor is defined by etching the disposed conductive layer 404 into adesired pattern of conductive paths. A typical electrical circuit forthe sensor 400 comprises two or more adjacent conductive paths withregions at a proximal end to form contact pads and regions at a distalend to form sensor electrodes. An electrically insulating cover layer406 such as a polymer coating can be disposed on portions of the sensor400. Acceptable polymer coatings for use as the insulating protectivecover layer 406 can include, but are not limited to, non-toxicbiocompatible polymers such as silicone compounds, polyimides,biocompatible solder masks, epoxy acrylate copolymers, or the like. Inthe sensors of the present invention, one or more exposed regions orapertures 408 can be made through the cover layer 406 to open theconductive layer 404 to the external environment and to, for example,allow an analyte such as glucose to permeate the layers of the sensorand be sensed by the sensing elements. Apertures 408 can be formed by anumber of techniques, including laser ablation, tape masking, chemicalmilling or etching or photolithographic development or the like. Incertain embodiments of the invention, during manufacture, a secondaryphotoresist can also be applied to the protective layer 406 to definethe regions of the protective layer to be removed to form theaperture(s) 408. The exposed electrodes and/or contact pads can alsoundergo secondary processing (e.g. through the apertures 408), such asadditional plating processing, to prepare the surfaces and/or strengthenthe conductive regions.

In the sensor configuration shown in FIG. 1H, an analyte sensing layer410 is disposed on one or more of the exposed electrodes of theconductive layer 404. Typically, the analyte sensing layer 410 is anenzyme layer. Most typically, the analyte sensing layer 410 comprises anenzyme capable of producing and/or utilizing oxygen and/or hydrogenperoxide, for example the enzyme glucose oxidase. Optionally, the enzymein the analyte sensing layer is combined with a second carrier proteinsuch as human serum albumin, bovine serum albumin or the like. In anillustrative embodiment, an oxidoreductase enzyme such as glucoseoxidase in the analyte sensing layer 410 reacts with glucose to producehydrogen peroxide, a compound which then modulates a current at anelectrode. As this modulation of current depends on the concentration ofhydrogen peroxide, and the concentration of hydrogen peroxide correlatesto the concentration of glucose, the concentration of glucose can bedetermined by monitoring this modulation in the current. In a specificembodiment of the invention, the hydrogen peroxide is oxidized at aworking electrode which is an anode (also termed herein the anodicworking electrode), with the resulting current being proportional to thehydrogen peroxide concentration. Such modulations in the current causedby changing hydrogen peroxide concentrations can be monitored by any oneof a variety of sensor detector apparatuses such as a universal sensoramperometric biosensor detector or one of the variety of similar devicesknown in the art such as glucose monitoring devices produced byMedtronic Diabetes.

In embodiments of the invention, the analyte sensing layer 410 can beapplied over portions of the conductive layer or over the entire regionof the conductive layer. Typically the analyte sensing layer 410 isdisposed on the working electrode which can be the anode or the cathode.Optionally, the analyte sensing layer 410 is also disposed on a counterand/or reference electrode. Methods for generating a thin analytesensing layer 410 include brushing the layer onto a substrate (e.g. thereactive surface of a platinum black electrode), as well as spin coatingprocesses, dip and dry processes, low shear spraying processes, ink-jetprinting processes, silk screen processes and the like. In certainembodiments of the invention, brushing is used to: (1) allow for aprecise localization of the layer; and (2) push the layer deep into thearchitecture of the reactive surface of an electrode (e.g. platinumblack produced by a sputtering process). Typically, the analyte sensinglayer 410 is coated and or disposed next to one or more additionallayers. Optionally, the one or more additional layers includes a proteinlayer 416 disposed upon the analyte sensing layer 410. Typically, theprotein layer 416 comprises a protein such as human serum albumin,bovine serum albumin or the like. Typically, the protein layer 416comprises human serum albumin. In some embodiments of the invention, anadditional layer includes an analyte modulating layer 412 that isdisposed above the analyte sensing layer 410 to regulate analyte contactwith the analyte sensing layer 410. For example, the analyte modulatingmembrane layer 412 can comprise a glucose limiting membrane, whichregulates the amount of glucose that contacts an enzyme such as glucoseoxidase that is present in the analyte sensing layer. Such glucoselimiting membranes can be made from a wide variety of materials known tobe suitable for such purposes, e.g., silicone compounds such aspolydimethyl siloxanes, polyurethanes, polyurea cellulose acetates,Nation, polyester sulfonic acids (e.g. Kodak AQ), hydrogels or any othersuitable hydrophilic membranes known to those skilled in the art.

In certain embodiments of the invention, an adhesion promoter layer 414is disposed between the analyte modulating layer 412 and the analytesensing layer 410 as shown in FIG. 1H in order to facilitate theircontact and/or adhesion. In a specific embodiment of the invention, anadhesion promoter layer 414 is disposed between the analyte modulatinglayer 412 and the protein layer 416 as shown in FIG. 3 in order tofacilitate their contact and/or adhesion. The adhesion promoter layer414 can be made from any one of a wide variety of materials known in theart to facilitate the bonding between such layers. Typically, theadhesion promoter layer 414 comprises a silane compound. In alternativeembodiments, protein or like molecules in the analyte sensing layer 410can be sufficiently crosslinked or otherwise prepared to allow theanalyte modulating membrane layer 412 to be disposed in direct contactwith the analyte sensing layer 410 in the absence of an adhesionpromoter layer 414.

C. Typical System Embodiments of the Invention

A specific illustrative system embodiment consists of a glucose sensorcomprising a sputtered/PVD electrode composition as disclosed herein, atransmitter and receiver and a glucose meter. In this system, radiosignals from the transmitter can be sent to the pump receiver at regulartime periods (e.g. every 5 minutes) to provide real-time sensor glucose(SG) values. Values/graphs can be displayed on a monitor of the pumpreceiver so that a user can self monitor blood glucose and deliverinsulin using their own insulin pump. Typically the sensor systemsdisclosed herein can communicate with other medical devices/systems viaa wired or wireless connection. Wireless communication can include forexample the reception of emitted radiation signals as occurs with thetransmission of signals via RF telemetry, infrared transmissions,optical transmission, sonic and ultrasonic transmissions and the like.Optionally, the device is an integral part of a medication infusion pump(e.g. an insulin pump). Typically in such devices, the physiologicalcharacteristic values include a plurality of measurements of bloodglucose.

FIG. 2 provides a perspective view of one generalized embodiment ofsubcutaneous sensor insertion system that can be adapted for use withthe sensor electrodes disclosed herein and a block diagram of a sensorelectronics device according to one illustrative embodiment of theinvention. Additional elements typically used with such sensor systemembodiments are disclosed for example in U.S. Patent Application No.20070163894, the contents of which are incorporated by reference. FIG. 2provides a perspective view of a telemetered characteristic monitorsystem 1, including a subcutaneous sensor set 10 provided forsubcutaneous placement of an active portion of a flexible sensor 12, orthe like, at a selected site in the body of a user. The subcutaneous orpercutaneous portion of the sensor set 10 includes a hollow, slottedinsertion needle 14 having a sharpened tip 44, and a cannula 16. Insidethe cannula 16 is a sensing portion 18 of the sensor 12 to expose one ormore sensor electrodes 20 to the user's bodily fluids through a window22 formed in the cannula 16. The base is designed so that the sensingportion 18 is joined to a connection portion 24 that terminates inconductive contact pads, or the like, which are also exposed through oneof the insulative layers. The connection portion 24 and the contact padsare generally adapted for a direct wired electrical connection to asuitable monitor 200 coupled to a display 214 for monitoring a user'scondition in response to signals derived from the sensor electrodes 20.The connection portion 24 may be conveniently connected electrically tothe monitor 200 or a characteristic monitor transmitter 200 by aconnector block 28 (or the like) as shown and described in U.S. Pat. No.5,482,473, entitled FLEX CIRCUIT CONNECTOR, which is incorporated byreference.

As shown in FIG. 2 , in accordance with embodiments of the presentinvention, subcutaneous sensor set 10 may be configured or formed towork with either a wired or a wireless characteristic monitor system.The proximal part of the sensor 12 is mounted in a mounting base 30adapted for placement onto the skin of a user. The mounting base 30 canbe a pad having an underside surface coated with a suitable pressuresensitive adhesive layer 32, with a peel-off paper strip 34 normallyprovided to cover and protect the adhesive layer 32, until the sensorset 10 is ready for use. The mounting base 30 includes upper and lowerlayers 36 and 38, with the connection portion 24 of the flexible sensor12 being sandwiched between the layers 36 and 38. The connection portion24 has a forward section joined to the active sensing portion 18 of thesensor 12, which is folded angularly to extend downwardly through a bore40 formed in the lower base layer 38. Optionally, the adhesive layer 32(or another portion of the apparatus in contact with in vivo tissue)includes an anti-inflammatory agent to reduce an inflammatory responseand/or anti-bacterial agent to reduce the chance of infection. Theinsertion needle 14 is adapted for slide-fit reception through a needleport 42 formed in the upper base layer 36 and through the lower bore 40in the lower base layer 38. After insertion, the insertion needle 14 iswithdrawn to leave the cannula 16 with the sensing portion 18 and thesensor electrodes 20 in place at the selected insertion site. In thisembodiment, the telemetered characteristic monitor transmitter 200 iscoupled to a sensor set 10 by a cable 402 through a connector 24 that iselectrically coupled to the connector block 28 of the connector portion24 of the sensor set 10.

In the embodiment shown in FIG. 2 , the telemetered characteristicmonitor 400 includes a housing 206 that supports a printed circuit board208, batteries 210, antenna 212, and the cable 202 with the connector204. In some embodiments, the housing 206 is formed from an upper case214 and a lower case 216 that are sealed with an ultrasonic weld to forma waterproof (or resistant) seal to permit cleaning by immersion (orswabbing) with water, cleaners, alcohol or the like. In someembodiments, the upper and lower case 214 and 216 are formed from amedical grade plastic. However, in alternative embodiments, the uppercase 214 and lower case 216 may be connected together by other methods,such as snap fits, sealing rings, RTV (silicone sealant) and bondedtogether, or the like, or formed from other materials, such as metal,composites, ceramics, or the like. In other embodiments, the separatecase can be eliminated and the assembly is simply potted in epoxy orother moldable materials that is compatible with the electronics andreasonably moisture resistant. As shown, the lower case 216 may have anunderside surface coated with a suitable pressure sensitive adhesivelayer 218, with a peel-off paper strip 220 normally provided to coverand protect the adhesive layer 118, until the sensor set telemeteredcharacteristic monitor transmitter 200 is ready for use.

In the illustrative embodiment shown in FIG. 2 , the subcutaneous sensorset 10 facilitates accurate placement of a flexible thin filmelectrochemical sensor 12 of the type used for monitoring specific bloodparameters representative of a user's condition. The sensor 12 monitorsglucose levels in the body, and may be used in conjunction withautomated or semi-automated medication infusion pumps of the external orimplantable type as described in U.S. Pat. Nos. 4,562,751; 4,678,408;4,685,903 or 4,573,994, to control delivery of insulin to a diabeticpatient.

In the illustrative embodiment shown in FIG. 2 , the sensor electrodes10 may be used in a variety of sensing applications and may beconfigured in a variety of positions on a base structure and further beformed to include materials that allow a wide variety of functions. Forexample, the sensor electrodes 10 may be used in physiological parametersensing applications in which some type of biomolecule is used as acatalytic agent. For example, the sensor electrodes 10 may be used in aglucose and oxygen sensor having a glucose oxidase enzyme catalyzing areaction with the sensor electrodes 20. The sensor electrodes 10, alongwith a biomolecule or some other catalytic agent, may be placed in ahuman body in a vascular or non-vascular environment. For example, thesensor electrodes 20 and biomolecule may be placed in a vein and besubjected to a blood stream, or may be placed in a subcutaneous orperitoneal region of the human body.

In the embodiment of the invention shown in FIG. 2 , the monitor ofsensor signals 200 may also be referred to as a sensor electronicsdevice 200. The monitor 200 may include a power source, a sensorinterface, processing electronics (i.e. a processor), and dataformatting electronics. The monitor 200 may be coupled to the sensor set10 by a cable 402 through a connector that is electrically coupled tothe connector block 28 of the connection portion 24. In an alternativeembodiment, the cable may be omitted. In this embodiment of theinvention, the monitor 200 may include an appropriate connector fordirect connection to the connection portion 24 of the sensor set 10. Thesensor set 10 may be modified to have the connector portion 24positioned at a different location, e.g., on top of the sensor set tofacilitate placement of the monitor 200 over the sensor set.

As noted above, embodiments of the sensor elements and sensors can beoperatively coupled to a variety of other system elements typically usedwith analyte sensors (e.g. structural elements such as piercing members,insertion sets and the like as well as electronic components such asprocessors, monitors, medication infusion pumps and the like), forexample to adapt them for use in various contexts (e.g. implantationwithin a mammal). One embodiment of the invention includes a method ofmonitoring a physiological characteristic of a user using an embodimentof the invention that includes an input element capable of receiving asignal from a sensor that is based on a sensed physiologicalcharacteristic value of the user, and a processor for analyzing thereceived signal. In typical embodiments of the invention, the processordetermines a dynamic behavior of the physiological characteristic valueand provides an observable indicator based upon the dynamic behavior ofthe physiological characteristic value so determined. In someembodiments, the physiological characteristic value is a measure of theconcentration of blood glucose in the user. In other embodiments, theprocess of analyzing the received signal and determining a dynamicbehavior includes repeatedly measuring the physiological characteristicvalue to obtain a series of physiological characteristic values in orderto, for example, incorporate comparative redundancies into a sensorapparatus in a manner designed to provide confirmatory information onsensor function, analyte concentration measurements, the presence ofinterferences and the like.

FIG. 3 shows a schematic of a potentiostat that may be used to measurecurrent in embodiments of the present invention. As shown in FIG. 3 , apotentiostat 300 may include an op amp 310 that is connected in anelectrical circuit so as to have two inputs: Vset and Vmeasured. Asshown, Vmeasured is the measured value of the voltage between areference electrode and a working electrode. Vset, on the other hand, isthe optimally desired voltage across the working and referenceelectrodes. The current between the counter and reference electrode ismeasured, creating a current measurement (Isig) that is output from thepotentiostat.

Embodiments of the invention include devices which process display datafrom measurements of a sensed physiological characteristic (e.g. bloodglucose concentrations) in a manner and format tailored to allow a userof the device to easily monitor and, if necessary, modulate thephysiological status of that characteristic (e.g. modulation of bloodglucose concentrations via insulin administration). An illustrativeembodiment of the invention is a device comprising a sensor inputcapable of receiving a signal from a sensor, the signal being based on asensed physiological characteristic value of a user; a memory forstoring a plurality of measurements of the sensed physiologicalcharacteristic value of the user from the received signal from thesensor; and a display for presenting a text and/or graphicalrepresentation of the plurality of measurements of the sensedphysiological characteristic value (e.g. text, a line graph or the like,a bar graph or the like, a grid pattern or the like or a combinationthereof). Typically, the graphical representation displays real timemeasurements of the sensed physiological characteristic value. Suchdevices can be used in a variety of contexts, for example in combinationwith other medical apparatuses. In some embodiments of the invention,the device is used in combination with at least one other medical device(e.g. a glucose sensor).

An illustrative system embodiment consists of a glucose sensor, atransmitter and pump receiver and a glucose meter. In this system, radiosignals from the transmitter can be sent to the pump receiver every 5minutes to provide real-time sensor glucose (SG) values. Values/graphsare displayed on a monitor of the pump receiver so that a user can selfmonitor blood glucose and deliver insulin using their own insulin pump.Typically, an embodiment of device disclosed herein communicates with asecond medical device via a wired or wireless connection. Wirelesscommunication can include for example the reception of emitted radiationsignals as occurs with the transmission of signals via RF telemetry,infrared transmissions, optical transmission, sonic and ultrasonictransmissions and the like. Optionally, the device is an integral partof a medication infusion pump (e.g. an insulin pump). Typically in suchdevices, the physiological characteristic values include a plurality ofmeasurements of blood glucose.

While the analyte sensor and sensor systems disclosed herein aretypically designed to be implantable within the body of a mammal, theinventions disclosed herein are not limited to any particularenvironment and can instead be used in a wide variety of contexts, forexample for the analysis of most in vivo and in vitro liquid samplesincluding biological fluids such as interstitial fluids, whole-blood,lymph, plasma, serum, saliva, urine, stool, perspiration, mucus, tears,cerebrospinal fluid, nasal secretion, cervical or vaginal secretion,semen, pleural fluid, amniotic fluid, peritoneal fluid, middle earfluid, joint fluid, gastric aspirate or the like. In addition, solid ordesiccated samples may be dissolved in an appropriate solvent to providea liquid mixture suitable for analysis.

EXAMPLES

Common acronyms used in the examples include: WE Working Electrode; GOxGlucose Oxidase; HSA Human Serum Albumin; SITS Sensor In-vitro TestSystem; GLM Glucose Limiting Membrane (an embodiment of an analytemodulating layer); OQ Operational Qualification; SAR Surface Area Ratio;BTS Bicarbonate Test System; and EIS Electrochemical ImpedanceSpectroscopy. The BTS and SITS tests discussed in the example are testsused to evaluate aspects of sensor performance. SITS measures sensorsignal in glucose solutions over 5-7 days, as wells as sensor oxygenresponse, temperature response, background current, linearity,stability, acetaminophen interference and response time. Dog tests areused to evaluate glucose sensor performance in vivo (Isig and calculatedblood glucose level) in diabetic and non-diabetic dogs for up to 3 daysand compares glucose level measured by continuous glucose sensors tothat measured by a glucose meter.

It is to be understood that this invention is not limited to theparticular embodiments described, as such may, of course, vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting, since the scope of the present invention will be limitedonly by the appended claims. In the description of the preferredembodiment, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration a specificembodiment in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

The descriptions and specific examples, while indicating someembodiments of the present invention are given by way of illustrationand not limitation. Many changes and modifications within the scope ofthe present invention may be made without departing from the spiritthereof, and the invention includes all such modifications.

Example 1: Sputtering Apparatus

FIG. 4 illustrates an apparatus comprising a chamber 400 for depositingmaterial (e.g., a thin film 402) using sputtering. A sputtering gas 404in the chamber 400 is ionized to form a plasma comprising ionized gasparticles 406 (e.g., Ar⁺). The ionized particles 406 bombard thesputtering target 408 comprising a metal composition. Collision of theionized particles 406 with the sputtering target 408 knocks off material410 (e.g., sputtered target atom) comprising the metal composition andaccelerates 412 the material 410 to the target surface on the substrate414, thereby forming a film 402 on the substrate 414. The ionized gasparticles 406 are accelerated towards target using electric and/ormagnetic fields applied by electrodes biased with voltage U⁻. Theparticle collision is controlled by process power (i.e., power of theelectric and/or magnetic fields until arrival of the ionized gasparticles on the sputtering gas) and pressure and composition of thesputtering gas (or ionized gas particle composition and pressure).

Example 2: Sputtering Conditions for Controlling Adhesion

The following deposition conditions may impact adhesion.

-   -   High pressure deposition conditions may cause the deposited film        to form under stress, leading to poor adhesion.    -   Deposition power may impact adhesion since higher deposition        rates may induce void pockets, leading to poor adhesion.    -   High temperatures used during deposition may evaporate any        remaining adsorbed water from the surfaces, improving adhesion.    -   Thicker films can generate stresses and worsen the adhesion.    -   Geometric area may also impact adhesion, and may be controlled        by the formation of pillars at the interface between the film        and the substrate.

In the experiments described herein, sputtering parameters includingpressure, power, temperature, and thickness, and combinations of theseparameters, were adjusted to determine their impact on adhesion and todetermine the parameters/parameter values that achieve optimal adhesionfor electrode processing. In one or more embodiments, the target foradhesion (or optimal adhesion) is strong enough to maintain adhesion ofthe base polyimide to the substrate during laser cutting, but weakenough to allow the base polyimide to be removed from the substrate forsensor assembly.

FIG. 5 illustrates a test sample comprising a layer stack 500 on a glasssubstrate 502. The layer stack includes a gold (Au) layer 504 on theglass substrate, a chromium (Cr) layer 506 on the Au layer 504, and basepolyimide layer 508 on the Cr layer 506.

FIG. 6A illustrates different patterns A-E of knife scratches or lasercutting marks applied to the layer stack 500 on the glass substrate 502that simulate the types of marks and cuts that may be applied duringprocessing of an electrode in a glucose sensor or other device.

FIG. 6B illustrates the patterns 600 applied to a silver layer on aglass substrate, showing that adhesion of the silver to the glass is tooweak to allow reproduction of the markings on the silver layer.

Using the patterns of markings illustrated in FIG. 6A, afeasibility-efficiency-compatibility study was performed to discover theimpact of metal (e.g., gold) sputtering conditions on the metal/glass(e.g., metal/glass) adhesion in the layer structure of FIG. 5 .

FIGS. 7A-7D illustrate how adhesion scores are assigned. FIG. 7Aillustrates a score of 0 is assigned when the patterns of FIG. 6A can beaccurately applied to the layer stack with highest quality andresolution of reproduction (representing the strongest adhesion of thelayer stack to the glass substrate). As the scores increase, theadhesion decreases and the patterns of markings are less well reproducedin the layer stack (FIGS. 7B and 7C). FIG. 7D illustrates a score of 10is assigned when the patterns of FIG. 6A cannot be accurately appliedto, and reproduced in, the layer stack 500 (representing the weakestadhesion of the layer stack to the glass substrate). This adhesion scoremethod is significantly less time consuming than performing a morequantitative analysis.

a. Experiment 1

The test samples of FIG. 6A were fabricated using the sputteringconditions of Table 1. The marking patterns of FIG. 6B were subsequentlyscratched/laser cut into each of the films on the test samples and anadhesion score was assigned to each reproduction, as shown in Table 1.

TABLE 1 Pressure Heat WHEN START Gold Adhesion Group (mTorr) Power (kW)(° C.) Time (min) Thickness (A) score 1 4 1.5 120 5 3703 0 2 4 0.2 12010 1270 0 3 4 1.5 No heat 5 3712 0 4 4 0.2 No heat 10 1209 0 5 100 1.5120 5 2304 0 6 100 0.2 120 10 685 3 7 100 1.5 No heat 5 2905 10 8 1000.2 No heat 10 671 4

FIGS. 7A-7D show the test results. The sputtered structure in FIG. 7Dwas fabricated using sputtering conditions including 100 mT pressure,1.6 kW power, a gold layer thickness of 897 Angstroms, and withoutheating.

The results show that the sputtering conditions for the samples 1-6 hadthe strongest adhesion (adhesion score 0) allowing accurate reproductionof the markings of FIG. 6B. FIGS. 7A-7D and Table 1 shows the surprisingand unexpected result that low pressure achieves extremely high adhesionand high pressure achieves low adhesion.

b. Experiment 2

FIG. 8B illustrates a test sample 1000 comprising an Au layer 1002including pillars 1004 at the interface between the Au layer 1002 andthe glass substrate 1006. Different test samples 1000 were fabricatedwith the Au layer 1002 deposited under different sputtering conditions(as shown in Table 2). FIG. 9 is a scanning electron microscope image ofthe pillared interface between the glass substrate 1006 and the goldlayer 1002.

The marking patterns of FIG. 6B were subsequently scratched/laser cutinto each of the Au films 1002 in the test samples 1000 using a knifeand an adhesion score was assigned to each reproduction, as shown inTable 2.

TABLE 2 Measured Pressure Target thickness Time thickness Adhesion Group(mTorr) Power (kW) without Cr (A) (sec) without Cr (A) score 1 55 1 5000532 4942 0 2 10 0.4 1000 239 1068 0 3 10 0.4 9000 2153 9232 0 4 10 1.61000 64 965 0 5 10 1.6 9000 580 8446 0 6 100 0.4 1000 700 952 8.5 7 1000.4 9000 6000 8622 10 8 100 1.6 1000 120 897 8 9 100 1.6 9000 1083 1080610 10 55 1 5000 532 4848 0 11 100 mT, 1.5 kW, 5 min 3952 by position!then 4 mT, 0.2 kW, 10 min

FIG. 5 illustrates fabrication of a backside counter electrodecomprising deposition of gold (Au) layer on a glass substrate,deposition of chromium (Cr) layer on the gold layer, deposition ofpolyimide including a base polyimide on the Cr layer, formation ofopenings in the polyimide, deposition of a Cr/Au stack inside theopenings, and peeling off the base polyimide together with the Au layerand Cr layer from the glass substrate.

It is to be understood that this invention is not limited to theparticular embodiments described, as such may, of course, vary. It isalso to be understood that the terminology used

The samples in Table 2 (samples 1-5 and 10) show low pressure sputteringachieves strong adhesion (low adhesion score). On the other hand, theresults for samples 6-9 show that sputtering at high pressure (above 55mTorr, e.g., 100 mTorr) achieves weak adhesion. A hypothesis is that thegold pillars decrease the contact area of gold/glass and increase theimpact of pressure on adhesion. The results also show a thicker film hasweaker adhesion to the glass substrate.

FIGS. 10A-10D illustrate the film of FIG. 6A including the gold pillars1004, fabricated using various sputtering conditions, after lasercutting with an example electrode pattern. FIGS. 12A and 12B illustratethe pattern is well reproduced in the film fabricated using 100 mTorrpressure, 0.4 W power, and a 952 Å thick gold layer (FIG. 10A) and inthe film fabricated using 100 mTorr pressure, 1.6 W power, and a 897 Åthick gold layer (FIG. 10B). FIGS. 10C and 10D illustrate the pattern isnot well reproduced in the film fabricated using 100 mTorr pressure, 0.4W power, and a 8922 Å thick gold layer (FIG. 10C) and in the filmfabricated using 100 mTorr pressure, 1.6 W power, and a 10806 Å thickgold layer (FIG. 10B). These results show that when high pressure isused, adhesion can be increased using a relatively thinner gold layer(adhesion decreases as gold layer thickness increases).

FIG. 11 illustrates a Pareto chart of the standardized effects ofchanging different factors (pressure, power, and gold thickness) onadhesion for the samples 500 fabricated using gold pillars 1004 at theinterface between the gold layer 504 and the glass substrate 502. In thePareto chart, response is the rate of changing pressure, sputteringpower, and gold layer thickness, and α=0.05 is a parameter used todetermine the statistically significant factors that control adhesion(in one or more examples, factors having a standardized effect onadhesion greater than α−0.05 are considered statistically significantfactors controlling adhesion).

FIG. 12 is a plot of the mean of the rate as a function of pressure,power, and gold thickness.

FIG. 13 is a contour plot of the rate versus thickness of the gold layerand sputtering pressure.

The DOE analysis (FIG. 11 , FIG. 12 , and FIG. 13 ) show that pressureis the dominant factor in controlling adhesion when pillars 1004 areformed at the interface. Specifically, the analysis shows higherpressure and a thicker (e.g., gold) layer in the film achieve weakeradhesion, and sputtering power has little effect on adhesion. A lowertemperature was found to provide weaker adhesion.

c. Effect of Two Layers of Gold on Adhesion

FIG. 14 illustrates another test sample comprising a layer stack 1400 ona glass substrate 1402. The layer stack 1400 includes a first gold layer1404 deposited using high pressure sputtering conditions on the glasssubstrate 1402, a second gold layer 1406 deposited using low pressuresputtering conditions on the first gold layer 1404, a chromium layer1408 sputtered on the second Au layer 1406, and base polyimide layer1410 deposited on the Cr layer 1408.

Following the previously discussed procedure, the marking patterns ofFIG. 6B were then scratched into each of the films 1400 in the testsamples using a knife or laser cutting. Table 2 compares the adhesionscore for the sample 1400 with the double gold layer 1406, 1404 (sample11) with the adhesion scores for samples 500 with a single gold layer504 deposited at a low or high pressure (samples 1-10).

FIG. 15A illustrates the film 500 of the test sample of FIG. 6Acomprising a gold layer 504 deposited using sputtering conditionsincluding 100 mTorr pressure, 1.5 kW power, for a duration of 5 minutes.FIG. 15B illustrates the film 1400 of the test sample of FIG. 14comprising a first gold layer 1404 deposited using sputtering conditionsincluding 100 mTorr pressure, 1.5 kW power, for a duration of 5 minutes,and the second gold layer 1406 deposited using sputtering conditionsincluding 4 mTorr pressure, 0.2 kW power, for a duration of 10 minutes.The results show that the test sample 1400 with the double gold layer(FIG. 14 ) has better adhesion than the sample 500 with one gold layer(FIG. 6A). Thus, the results unexpectedly and surprisingly show thecombination of high/low-pressure gold layers 1404, 1406 cansignificantly affect the adhesion.

FIGS. 15C, 15D, and 15E illustrate the film 1400 with two gold layers1404, 1406 deposited using the conditions for FIG. 15B has an adhesionthat varies depending on position on the surface area. The adhesionuniformity may be increased by reducing defects and dust on the glasssubstrate and improving uniformity of deposition in the sputteringapparatus.

Example 3: Controlling Sputtering Rate

A DOE analysis was performed to determine the process parametersaffecting sputtering rate of gold onto a glass substrate when no heat isapplied. FIG. 16 illustrates a Pareto chart of the standardized effectsof changing pressure, power, and gold thickness on sputtering rate, whenno heat is applied. In the Pareto chart, response is the sputtering ratein Angstroms per second and α=0.05.

FIG. 17 is a plot of the mean of the sputtering rate as a function ofpressure, power, and gold thickness.

FIG. 18 is a contour plot of the sputtering rate versus sputtering power(kW) and pressure (mTorr).

The DOE analysis (FIG. 16 , FIG. 17 , and FIG. 18 ) shows that anoptimal pressure exists for a maximum sputtering rate and thatsputtering rate increases linearly with sputtering power. Thus, asillustrated herein, PVD conditions can be carefully selected to increasesputtering rate and control adhesion. In one or more embodiments, theDOE analysis is used to determine the sputtering parameters that achievethe fastest deposition rate and desired adhesion. Power and pressure maybe used to control sputtering rate and adhesion.

Although the examples 2-4 refer to sputtering, the same results andfindings (including adhesion control by properly selecting pressure)apply to deposition using PVD generally (e.g., including, but notlimited to, electron beam deposition).

Example 4: Analyte Sensor Apparatus Fabrication

FIG. 19 , FIG. 20 , and FIG. 1D illustrate a method of making an analytesensor apparatus 100 d.

Block 1900 represents providing a base (e.g., rigid) substrate 2000(e.g., a glass substrate).

Block 1902 represents depositing metal 2002 a, 2002 b (physical vapordeposited metal) on the base substrate, e.g., using PVD. In one or moreembodiments, the metal comprises a first layer 2002 a (e.g., Au layer)on the base substrate 2000 and a second layer (e.g., Cr or Ti layer)2002 b on first Au layer 2002 a. In one or more examples, the metal 2002a, 2002 b extends laterally so as to form contact pads 110, 114.

Example PVD conditions include a pressure in a range of 2-250 mTorr,70-100 mTorr, or 50-125 mTorr, a power in a range of 10 W-100 kW (e.g.,0.5 kW-2 kW, e.g., 0.8 kW) and a thickness of each of the metal layersin a range of at least 100 Angstroms (e.g., 1000-9000 Å). The PVD stepscan comprise the pressure control steps described herein, e.g., thesteps of Blocks 2600-2604 in FIG. 26 of Example 9. Example PVD processesinclude, but are not limited to, sputtering and electron beamdeposition.

Block 1904 represents depositing a first insulation layer 2004 on themetal 2002 a, 2002 b. Example insulation layers include, but are notlimited to, a polymer layer (such as, but not limited to, a polyimide).

Block 1906 represents depositing and patterning second metal 2006 a,2006 b on the first insulation layer 2004, 104 b. In one or moreexamples, second metal comprises a two layers—a second layer 2006 bcomprising Au on a first layer 2006 b comprising Cr or Ti) and extendslaterally so as to form contact pads 110, 114.

Block 1908 represents depositing a second insulation layer 2008, 118onto the first insulation layer 2004 and the second metal 2006 a, 2006 bon the first insulation layer 2004. Example insulation layers include,but are not limited to, a polymer layer (such as, but not limited to, apolyimide).

Block 1910 represents forming a first opening 2010 a and a secondopening 2010 b in the second insulation layer 2004 so as to exposesecond metal 2006 b.

Block 1912 represents depositing third metal into the first opening 2010a and onto the second metal 2010 b so as to form a working electrode WE(see FIG. 1D).

Block 1914 represents depositing fourth metal into the second opening2010 b and onto second metal 2006 b so as to form a reference electrode(RE) (see FIG. 1D).

Block 1916 represents additional steps, including formation of openingsin the second insulation layer 118 to expose the metal contact pads 110,114 b (referring to FIG. 1D) which comprise second metal 2006 a, 2006 b,and curing if necessary.

Block 1918 represents defining the analyte sensors in the film 2012comprising the metal 2002 a, 2002 b, the second metal 2006 a, 2006 b,the first insulation layer 2004, 104 b, the second insulation layer2008, 118, and the electrodes WE, RE.

Block 1920 represents removing 2014 (e.g., peeling) the analyte sensors100 d from the base substrate 2000. In one or more embodiments, the stepcomprises removing (e.g., peeling) the physical vapor deposited metal2002 a, 2002 b from the substrate 2000.

Block 1922 represents the end result, a sensor apparatus, e.g., asillustrated in FIG. 1D. The metal layers 2002 a, 2002 b, CE act as thebackside counter electrode BCE as well as a layer for controllingadhesion to the base substrate 2000 using the pressure control methodsdescribed herein (see e.g., examples 2-3). Base polyimide layer 2004,104 b does not require patterning or etching to contact the BCE. In oneor more examples, the method of Example 4 enables fabrication of adevice comprising 1 flex with electrodes on both sides (as compared to acontrol device having interdigitated electrodes on one side asillustrated in FIG. 1F). As illustrated herein, a plurality (e.g., atleast 36) of sensors 100 d removed from the base substrate can allexhibit an ISIG of within 15% (see, e.g., FIG. 21D).

Example 5: Sits Results for Unity Sensor of Example 4

FIGS. 21A-21C illustrate SITS results for a control sensor asillustrated in FIG. 1F and FIG. 21D-21F illustrate the SITS results forthe sensor of FIG. 1G (simulating/representing the performance of thedevice of FIG. 1D having a BCE fabricated using the method Example 4).

The sensor of FIG. 1G has two flexes:

-   -   Flex 1: nominal electrode E3 with tape over CE contact pad at        the transmitter connection. The tape does not contact the body.    -   Flex 2: nominal E3 layer comprising base polyimide and nominal        E3 electrode comprising Cr/Au and tape over WE & RE contact pad        regions at the transmitter connection. This flex is a nominal E3        flex manufactured only through the metal sputtering process and        the tape does not contact the body.

While the sensor of FIG. 1D has a single flex including the CE, the WE,and the RE, the performance of the FIG. 1D device is expected to besimilar to the performance of the two flex FIG. 1G device because boththe FIG. 1D device and the FIG. 1G device have a CE electrode on thebackside on the opposite side from the WE.

TABLE 3 SITS summary for testing of the device of FIG. 1D for 3 totalSITS runs. Day 7/8 Isig Day 7/8 Isig Long Term Long Term VariablilityVariablility Stability Stability (0.1% O2) (0.1% O2) (5% O2) (5% O2)SITS Run Control BCE Control BCE S3164 13.8 4.63 13.13% 12.04% S316611.95 7.49 −0.37 1.86 S3167 5.78 4.75 16.70% 4.24% * indicatesstatistically significant difference. The number of devices tested was n= 36 for the BCE device of FIG. 1D and n = 36 for the control device).

For the data in FIGS. 21A-21C, the working electrode 132 and counterelectrode 134 in the control sensor 130 comprise Pt, and the referenceelectrode in the control sensor 130 comprises Ag/AgCl. For the data inFIGS. 21D-21F, the WE in the sensor of FIG. 1G comprises Pt, the CE inthe sensor of FIG. 1G comprises Au, and the RE in the sensor of FIG. 1Gcomprises Ag/AgCl.

The data in FIGS. 21D-21F and Table 3 for sensors tested in vivo on apig illustrate improved long-term stability over the entire test,eliminated overnight Isig drift, and significantly (and surprisingly)less sensor to sensor variability (especially at low O₂concentrations—stress conditions) for the BCE device of FIG. 1G(representing the device of FIG. 1D), as compared to the control sensorof FIG. 1F. In addition, the BCE of FIG. 1G did not exhibit majordifferences in temperature and AC responses, and there were no negativeobservations from visual inspection.

FIGS. 21C and 21F also show Vcounter (Vcntr) activity/movement inresponse to glucose sensing using the device of FIG. 1G is alsosurprisingly reduced as compared to for the control sensor of FIG. 1F.Moreover, the data shows Vcounter for the FIG. 1G sensor appeared morestable with a lower steady-state voltage.

Example 6: Analyte Sensor Apparatus Fabrication

FIG. 22 is a flowchart illustrating a method of fabricating a glucosesensor or sensor flex (referring also to FIGS. 1A-1D and FIG. 23 ). Themethod comprises the following steps.

Block 2200 represents depositing one or more metal layers on a (e.g.,rigid) substrate 2302 (e.g., glass) using physical vapor deposition(e.g., sputtering or electron beam deposition). Example metal layers2300 a, 2300 b include, but are not limited to, Au, Cr, Ti andcombinations thereof. In one or more embodiments, the layers 2300 a,2300 b comprise one or more gold layers deposited on a glass substrate2302 followed by deposition of Cr on the gold layer(s).

Example PVD conditions include a pressure in a range of 2-250 mTorr,70-100 mTorr, or 50-125 mTorr, a power in a range of 10 W-100 kW (e.g.,0.5 kW-2 kW, e.g., 0.8 kW) and a thickness of each of the metal layers2300 a, 2300 b of at least 100 Angstroms (e.g., 1000-9000 Å). The PVDsteps can comprise the pressure control steps described herein, e.g.,the steps of Blocks 2600-2604 in FIG. 26 of Example 9.

Block 2202 represents depositing a first or base layer 116 on thesputtered metal layer(s) 2300 a, 2300 b formed in Block 2200. Examplebase layers include, but are not limited to, a polymer layer (such as,but not limited to, a polyimide forming a first or base polyimidelayer). In one or more embodiments, the step comprises spin casting thepolymer (e.g., polyimide) onto the metal layer(s) 2300 a, 2300 b andthen pre-curing the polymer (e.g., polyimide).

Block 2204 represents optionally patterning and/or etching the baselayer 116 for deposition of one or more electrodes (e.g., WE and RE)and/or one or more contact pads 114. In one or more examples, thepatterning comprises depositing a dry-etch mask (e.g., photoresist dryetch mask) on the base layer 116, dry etching the base layer 116 throughopenings in the dry-etch mask, and stripping the dry-etch mask from thebase layer 116, thereby forming an etched pattern (including firstopening) in the base layer 116.

Block 2206 represents depositing metal 112 (second metal) comprising theCE onto the etched pattern. Examples of the metal 112 include, but arenot limited to, Au, Ti, and Cr and combinations thereof (e.g., Au and Tiand/or Cr). In one or more examples, the step comprises sputtering orelectron beam depositing the metal 112 onto the base layer 116 includingthe etched pattern; depositing a mask (e.g., photoresist wet-etch mask)on the metal 112 deposited onto the base layer 116; etching (e.g., wetetching) the metal through openings in the mask; and stripping the maskfrom the metal 112.

Block 2208 represents depositing an insulation layer 104 a (firstinsulation layer) on the base layer 116 and the metal on 112 the baselayer 116. Example insulation layers include, but are not limited to, apolymer layer (such as, but not limited to, a polyimide forming a firstinsulation polyimide layer). In one or more examples, the insulationlayer 104 a is blanket deposited on the metal 112. In one or morefurther examples, the depositing comprises spin casting the insulationlayer 104 a so as to cover the base layer 116 and the metal 112; andpre-curing the insulation layer 104 a.

Block 2210 represents depositing and patterning metal 108 (third metal)onto the first insulation layer 104 a. Examples of metal include Au, Ti,and Cr and combinations thereof (e.g., Au and Ti and/or Cr). In one ormore examples, the step comprises sputtering/e-beam depositing a film(e.g., thin film) of the metal 108 onto the first insulation layer 104 aso as to blanket cover the first insulation layer 104 a; depositing amask (e.g., photoresist wet-etch mask) on the metal sputtered onto thefirst insulation layer 104 a etching (e.g., wet etching) the metalthrough openings in the mask; and stripping the mask from the metal 108.

Block 2212 represents depositing a second insulation layer 118 on thefirst insulation layer 104 a and the metal 108 on the first insulationlayer 104 a. Example second insulation layers include, but are notlimited to, a polymer layer (such as, but not limited to, a polyimideforming a second insulation polyimide layer). In one or more examples,the step comprises spin casting the second insulation layer 118 onto thefirst insulation layer 104 a and the metal 108 on the first insulationlayer 104 a; and pre-curing the second insulation layer 118.

Block 2214 represents patterning the second insulation layer 118, e.g.,using photolithography, and forming an etched pattern in the secondinsulation layer 118 comprising a second well or second opening 2304 anda third well or third opening 2306.

Block 2216 represents optionally performing a final cure of thestructure formed in blocks 2200-2214.

Block 2218 represents optionally removing residue from the secondinsulation layer 118, e.g., using O₂.

Block 2220 represents depositing metal (fourth metal) and other layersneeded to form the WE. In one or more embodiments, the step comprisesdepositing metal pillars 124 into the second well/opening 2304 formed inthe second insulation layer 118. Examples of metal pillars include, butare not limited to, platinum or gold pillars. In one or moreembodiments, the step comprises depositing a photoresist lift off maskin the first well 2304; performing a cleaning (e.g., 02 plasma descum)of the photoresist lift off mask; sputtering metal into openings in themask so as to form the metal pillars 124 extending through the openingsfrom the exposed surface of the metal 108 in the first well 2304; andlifting off/removing the mask, leaving the pillars 124 on the metal 108.

Block 2222 represents depositing metal (fifth metal) into the thirdwell/opening 2306 so as to form the reference electrode (RE) in thethird well or third opening 2306. Examples of deposition methodsinclude, but are not limited to, depositing the metal usingelectroplating or screen printing. Example metal for the RE comprises,but is not limited to, Pt, gold, and Cr.

Block 2224 represents performing a chemistry step, wherein additionalchemically active layers/constituents are deposited on the WE (e.g.,onto the pillars) so that the WE has the proper functionality in aglucose sensor. Example constituents include, but are not limited to,one or more of an interference rejection constituent, an analyte sensingconstituent 410, a protein constituent 416, an adhesion promoting layer414, and an analyte modulating layer 412, and/or cover layer asdescribed herein.

Block 2226 represents processing the structure into individual sensors100, e.g., by cutting or laser patterning.

Block 2228 represents singulation or removing (e.g., peeling) theindividual analyte sensors 100 a-d from substrate 2302. In one or moreembodiments, the PVD methods described herein directed to adhesioncontrol enable singulation of the flex or sensor 100 a-d from thesubstrate 2302 (e.g., glass) without damaging the CE and the contactpads 110, 114. In one or more embodiments, the step comprises removing(e.g., peeling) the physical vapor deposited metal 2300 a, 2300 b fromthe (e.g., rigid) substrate 2002.

Block 2230 represents the end result, an analyte sensor apparatus 100a-d, such as a glucose sensor, as illustrated in FIGS. 1A-1D. FIGS.1A-1D illustrate various double-sided single flex sensor embodimentsaccommodating multiple electrodes and including electrodes on both sidesof the sensor flex 100 a-d, and wherein components of the sensor 100 a-dare flexible so as to form a flexible sensor (sensor flex). The flex orsensor 100 a-100 d includes the WE and RE on the top side of flex orsensor and a CE on the backside of the flex or sensor. In one or moreembodiments, a smooth CE is formed on the backside 102 b and has enoughsurface area to balance the electrochemical reaction occurring at theWE. However, in one or more examples, no chemistry needs to take placeon the backside 102 b of the flex or sensor 100 a-d. Extra electrodes(not shown) may also be included for a background sensor or differentialsensor etc., and the electrodes may interface with a transmitterconnection scheme. The device 100 a-d can be used in the potentiostatcircuit of FIG. 3 .

In one or examples, the fabrication method described herein may increasethe working electrode area, prevent the “drift” effect and/or simplifythe manufacturing process.

Investigation of the process parameters has found excellent processcontrol, design control and repeatability. The process is a highthroughput process and easily transferable between plates and 8″ wafer.

Example 7: Method of Depositing a Film and Controlling Adhesion

FIG. 24 is a flowchart illustrating a method of depositing films on asubstrate. The method comprises the following steps.

Block 2400 represents controlling pressure of a gas in a chamber usedfor depositing metal using physical vapor deposition (PVD). In one ormore examples, the step additionally comprises controlling at least oneadditional PVD parameter selected from thickness of the metal, a numberof layers of the metal, and a power used during the physical vapordeposition.

Block 2402 represents depositing the metal on a substrate using physicalvapor deposition (PVD).

Block 2404 represents depositing a film on the metal.

Block 2406 represents measuring the degree of adhesion of the film tothe substrate as a function the at least one PVD parameter (includingpressure). In one or more embodiments, the measuring comprises assigningan adhesion score.

Example PVD conditions include a pressure in a range of 2-250 mTorr,70-100 mTorr, or 50-125 mTorr, a power in a range of 10 W-100 kW (e.g.,0.5 kW-2 kW, e.g., 0.8 kW) and a thickness of each of the metal layersin a range of at least 100 Angstroms (e.g., 1000-9000 Å).

Block 2408 represents optionally determining the pressure or other PVDparameter that achieves a desired adhesion of the film to the substrate.In one or more examples, the step comprises analyzing the degree ofadhesion as a function of the at least one physical vapor depositionparameter so as to determine the relative impact of the at least onephysical vapor deposition parameter on the degree of adhesion. In one ormore examples, the analyzing comprises performing a design ofexperiments (DOE) analysis; and plotting the degree of adhesion as aresponse in a Pareto chart. The adhesion score and determining/analyzingsteps of block 2408 may be performed in a processor or computer using acomputer-readable program code having instructions, which when executed,cause the processor or computer to perform a statistical analysis of themeasurements obtained in Block 2406, thereby determining the PVDparameter that achieves the desired adhesion.

Example 8: Method of Making a Device

FIG. 25 is a flowchart illustrating a method of depositing of a film ormaking a device on a substrate. The method comprises the followingsteps.

Block 2500 represents placing a substrate (e.g., rigid substrate) in aphysical vapor deposition (PVD) (e.g., sputtering) chamber.

Block 2502 represents setting a PVD conditions including pressure of agas in the chamber used for depositing material using PVD. In one ormore examples, the pressure is determined using the methods described inExample 7.

Block 2504 represents depositing PVD metal on the substrate using thephysical vapor deposition at the pressure.

In one or more embodiments, the metal comprises a plurality of layerseach deposited at a different pressure.

In one or more embodiments, the PVD comprises sputtering or electronbeam deposition, including ionizing the gas so as to form ionized gasparticles; and accelerating the ionized gas particles onto a targetcomprising the metal using an electric and/or magnetic field having apower in a range of, e.g., 10 Watts-100 kW (e.g., 0.5 kilowatts to 2kilowatts). In one or more examples, the pressure of the gas is in arange of 2-250 mTorr, 70-100 mTorr, or 50-125 mTorr. In one or moreembodiments, the PVD metal comprises one or more layers each having athickness in a range of at least 100 Angstroms, e.g., 1000-9000 Å. Inone or more examples, the PVD metal comprises a first layer deposited onthe substrate at the pressure in a range of 50-250 mTorr (or 5-150mTorr) and a second layer deposited on the first layer at the pressurein a range of 2-50 mTorr or 2-30 mTorr).

In one or more embodiments, the PVD deposited metal includes at leastone structured layer selected from a patterned layer, a roughened layer,a non-uniform layer, a layer including voids, and a layer comprisingpillars.

Block 2506 represents depositing a film or device structure on themetal, e.g., as described in Examples 4 and 6. The pressure selected inBlock 2602 may be associated with a pre-determined adhesion of the filmto the substrate, the pre-determined adhesion allowing (1) processing ofthe film into a device while the film is adhered to the substrate; and(2) removal (e.g., peeling) of the device from the substrate.

Block 2508 represents optionally processing the film into one or moredevices. In one or more examples, the processing comprises patterning orcutting the film.

Block 2510 represents optionally peeling or removing the devices fromthe substrate.

Block 2512 represents the end result, a device, e.g., as illustrated inFIGS. 1A-1D. In one or more embodiments, the device comprises an exposedsurface S of PVD metal 2302 a, 2302 b, 2002 a, 2002 b peeled/removedfrom a rigid substrate 2000, 2302. Example devices include, but are notlimited to, a device including a microelectromechanical (MEMS) devicestructure, an optoelectronic device structure, a circuit, a batteryelectrode, a fuel cell electrode, or an electrode CE having anelectrochemically active surface 122. Microarrays and multielectrodearrays may be fabricated.

As illustrated herein, investigation of the process parameters has foundexcellent process control, design control and repeatability. The processis a high throughput process and easily transferable between plates and8″ wafer.

In one or more examples, a separation D, arrangement, or configurationof the working electrode WE and the counter electrode CE in an analytesensor apparatus 100 a-100 e is such that, in response to a constantanalyte concentration, the electrical current (ISIG) varies by less than15% over a period of 31 days and/or the chemical products created by theworking and counter electrode reactions do not interfere or havedetrimental interactions with the performance of the electrodes (WE, CE)(see FIGS. 21D-21F).

In one or more examples, in a set of at least 36 of the sensors 100a-100 e fabricated using the methods described herein, a separation D,arrangement, configuration, and electroactivity of the working electrodeWE and the counter electrode CE in each of the sensors 100 a-100 e issuch that, in response to the same analyte concentration, the electricalcurrents (ISIG) outputted by each of the sensors are within 15% (seeFIGS. 21D-21F).

In one or more embodiments, the PVD apparatus is coupled to a processoror computer using a computer-readable program code having instructions,which when executed, cause the processor or computer to control the PVDdeposition parameters in PVD apparatus, so as to achieve a desiredadhesion of the film to the substrate.

It is to be understood that this invention is not limited to theparticular embodiments described, as such may, of course, vary. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting, since the scope of the present invention will be limitedonly by the appended claims. In the description of the preferredembodiment, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration a specificembodiment in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

The descriptions and specific examples, while indicating someembodiments of the present invention are given by way of illustrationand not limitation. Many changes and modifications within the scope ofthe present invention may be made without departing from the spiritthereof, and the invention includes all such modifications.

What is claimed is:
 1. An analyte sensor apparatus, comprising: aworking electrode; a counter electrode; an insulation layer between theworking electrode and the counter electrode, wherein: the workingelectrode is spatially separated from the counter electrode by adistance of at least 1 micrometer, the working electrode comprises ametal composition having an electroactive surface, and the workingelectrode and the counter electrode are non-interdigitated; and ananalyte sensing layer on the working electrode, wherein the analytesensing layer detectably alters electrical current at the workingelectrode in a presence of an analyte.
 2. The apparatus of claim 1,wherein the working electrode and the counter electrode are on a sameside of the analyte sensor apparatus comprising a glucose sensor.
 3. Theapparatus of claim 1, wherein: the working electrode is on a first sideof the insulation layer; and the counter electrode on a second side ofthe insulation layer opposite the first side.
 4. The apparatus of claim3, further comprising: a sensing portion and a connection portion, theconnection portion including at least one of a first contact pad or asecond contact pad; the sensing portion including the working electrodeand the counter electrode; and the sensing portion being flexible so asto allow formation of a fold between the sensing portion and theconnection portion.
 5. The apparatus of claim 3, wherein the counterelectrode comprises a second metal composition having an adhesion thatallows peeling of the second metal composition from a direct contactwith a substrate so as to remove the analyte sensor apparatus from thesubstrate on which the analyte sensor apparatus was fabricated.
 6. Theapparatus of claim 5, further comprising: the second metal compositioncomprising a surface exposed after being peeled from the substrate; areference electrode on the first side of the insulation layer;insulation between the reference electrode and the working electrode;first contact metal electrically contacting the working electrode, thefirst contact metal comprising a first contact pad; second contact metalelectrically contacting the counter electrode, the second contact metalcomprising a second contact pad; and wherein: the insulation layer andthe insulation comprise polyimide, and the working electrode, thecounter electrode, the insulation layer, the insulation, and the analytesensing layer are flexible.
 7. The analyte sensor apparatus of claim 5,wherein the second metal composition comprises at least one structuredlayer selected from a patterned layer, a roughened layer, a non-uniformlayer, a layer including voids, and a layer comprising pillars, the atleast one structure layer controlling the adhesion to the substrate. 8.The apparatus of claim 7, wherein the analyte sensor apparatus comprisesa glucose sensor.
 9. The analyte sensor apparatus of claim 7, whereinthe counter electrode comprises physical vapor deposited (PVD) metal.10. The analyte sensor apparatus of claim 9, wherein the second metalcomposition comprises gold.
 11. A method of making an analyte sensorapparatus, comprising depositing a working electrode and a counterelectrode so that an insulation layer is between the working electrodeand the counter electrode, wherein: the working electrode is spatiallyseparated from the counter electrode by a distance of at least 1micrometer, the working electrode comprises a metal composition havingan electroactive surface, and the working electrode and the counterelectrode are non-interdigitated; and depositing an analyte sensinglayer on the working electrode, wherein the analyte sensing layerdetectably alters electrical current at the working electrode in apresence of an analyte.
 12. The method of claim 11, further comprisingdepositing the working electrode and the counter electrode on a sameside of the analyte sensor apparatus comprising a glucose sensor. 13.The method of claim 11, further comprising: depositing the workingelectrode is on a first side of the insulation layer; and depositing thecounter electrode on a second side of the insulation layer opposite thefirst side.
 14. The method of claim 12, further comprising: providing asensing portion and a connection portion, the connection portionincluding at least one of a first contact pad or a second contact pad;wherein: the sensing portion includes the working electrode and thecounter electrode; and the sensing portion is flexible so as to allowformation of a fold between the sensing portion and the connectionportion.
 15. The method of claim 11, further comprising depositing thecounter electrode comprising a second metal composition having anadhesion that allows peeling of the second metal composition from adirect contact with a substrate so as to remove the analyte sensorapparatus from the substrate on which the analyte sensor apparatus wasfabricated.
 16. The method of claim 15, further comprising peeling theanalyte sensor apparatus from the substrate.
 17. The method of claim 15,wherein the second metal composition comprises a surface exposed afterbeing peeled from the substrate, the method further comprising:depositing a reference electrode on the first side of the insulationlayer; providing insulation between the reference electrode and theworking electrode; depositing first contact metal electricallycontacting the working electrode, the first contact metal comprising afirst contact pad; depositing second contact metal electricallycontacting the counter electrode, the second contact metal comprising asecond contact pad; and wherein: the insulation layer and the insulationcomprise polyimide, and the working electrode, the counter electrode,the insulation layer, the insulation, and the analyte sensing layer areflexible.
 18. The method of claim 15, wherein the second metalcomposition comprises at least one structured layer selected from apatterned layer, a roughened layer, a non-uniform layer, a layerincluding voids, and a layer comprising pillars, the at least onestructure layer controlling the adhesion to the substrate.
 19. Themethod of claim 15, further comprising depositing the second metalcomposition using physical vapor deposition.
 20. The method of claim 11,wherein the analyte sensor apparatus comprises a glucose sensor.